I've Been Killing Slimes for 300 Years and Maxed Out My Level

I've Been Killing Slimes for 300 Years and Maxed Out My Level (スライム倒して300年、知らないうちにレベルMAXになってました) is a Japanese isekai light novel series written by Kisetsu Morita and illustrated by Benio. It was serialized online since 2016 on the user-generated novel publishing website Shōsetsuka ni Narō and, in the following year, it was acquired by SB Creative, who published the first light novel volume in 2017. The light novel has been licensed in North America by Yen Press, who published the first volume in April 2018. It has received a manga adaptation by Yūsuke Shiba in the same year as its debut of the light novel in Square Enix's Gangan Online website, also licensed by Yen Press. An anime television series adaptation by Revoroot aired from April to June 2021. A second season by Teddy is set to premiere in 2025. The series has also spawned various spin-off novels.

Plot
After living a painful life as an office worker on Earth, Azusa ends up dying from overworking. She finds herself reincarnated as an undying, unaging witch in a new world, and now she vows to spend her days as pleasantly and stress-free as possible. She makes a living by hunting slimes, the easiest targets. But after centuries of doing this simple job, she has become extremely powerful and finds that she can no longer maintain her low-key lifestyle.

Characters

 * Azusa Aizawa (アズサ・アイザワ / 相沢梓)
 * A former Japanese office worker reincarnated as an immortal witch, called "the Witch of the Highlands" (高原の魔女). Weary of any overwork and excitement following her untimely death in her old life, she strives for an easy tune of existence, but once word gets out of her unprecedented prowess, she finds herself beleaguered by challengers trying to prove themselves on her, or all sorts of people willing to join her. However, she is compassionate and in time becomes very protective of her adopted family, much to the chagrin of anyone who deliberately endangers them.
 * A former Japanese office worker reincarnated as an immortal witch, called "the Witch of the Highlands" (高原の魔女). Weary of any overwork and excitement following her untimely death in her old life, she strives for an easy tune of existence, but once word gets out of her unprecedented prowess, she finds herself beleaguered by challengers trying to prove themselves on her, or all sorts of people willing to join her. However, she is compassionate and in time becomes very protective of her adopted family, much to the chagrin of anyone who deliberately endangers them.


 * Laika (ライカ)
 * A roughly 300-year-old shape-shifting red dragon girl. Originally arrogant, she came to Azusa to fight her and prove herself to be stronger than her. After getting soundly beaten by Azusa, she joins her as her first apprentice and housekeeper, eventually becoming her adoptive younger sister. She has a rivalry with Flatorte.
 * A roughly 300-year-old shape-shifting red dragon girl. Originally arrogant, she came to Azusa to fight her and prove herself to be stronger than her. After getting soundly beaten by Azusa, she joins her as her first apprentice and housekeeper, eventually becoming her adoptive younger sister. She has a rivalry with Flatorte.


 * Falfa (ファルファ)
 * An about 50-year-old slime spirit girl, and the elder of twin sisters. She is one of Azusa's adoptive daughters. Both she and her sister Shalsha are reincarnations of the slime creatures that Azusa killed for a living; but while Shalsha is initially deeply resentful towards Azusa at first, Falfa adores her and considers her their "mother".
 * An about 50-year-old slime spirit girl, and the elder of twin sisters. She is one of Azusa's adoptive daughters. Both she and her sister Shalsha are reincarnations of the slime creatures that Azusa killed for a living; but while Shalsha is initially deeply resentful towards Azusa at first, Falfa adores her and considers her their "mother".


 * Shalsha (シャルシャ)
 * Falfa's younger slime spirit twin sister. She is one of Azusa's adoptive daughters. Driven by feelings of revenge against Azusa for killing all the slimes, she learned a type of magic which is effective against one specific race only, but in turn is potent enough to counter even Azusa's magic; she is also very learned in magic lore. After being very easily beaten by Laika, Shalsha and Falfa join Azusa's household upon her invitation. In turn, she abandoned her desire for revenge upon learning that she is not the only one targeting slimes. She is very inquisitive, and habitually takes notes of anything new she comes across.
 * Falfa's younger slime spirit twin sister. She is one of Azusa's adoptive daughters. Driven by feelings of revenge against Azusa for killing all the slimes, she learned a type of magic which is effective against one specific race only, but in turn is potent enough to counter even Azusa's magic; she is also very learned in magic lore. After being very easily beaten by Laika, Shalsha and Falfa join Azusa's household upon her invitation. In turn, she abandoned her desire for revenge upon learning that she is not the only one targeting slimes. She is very inquisitive, and habitually takes notes of anything new she comes across.


 * Halkara (ハルカラ)
 * A young (about 225-year-old) female elf apothecary woman from Frant Province with very abrupt mood swings and a voluptuous build. She invented an extremely popular, alcohol-based energy drink; but when the demon lord Beelzebub drank from it, she became sick (from overworking after getting addicted to it) and then went after Halkara, whereupon the elf, believing that Beelzebub wanted in fact revenge, fled to Azusa for protection. After being given shelter, and after the misunderstanding with Beelzebub is cleared, Halkara decides to stay with Azusa and restart the production of her energy drink. She is very knowledgable about mushrooms, but scatterbrained and accident-prone.
 * A young (about 225-year-old) female elf apothecary woman from Frant Province with very abrupt mood swings and a voluptuous build. She invented an extremely popular, alcohol-based energy drink; but when the demon lord Beelzebub drank from it, she became sick (from overworking after getting addicted to it) and then went after Halkara, whereupon the elf, believing that Beelzebub wanted in fact revenge, fled to Azusa for protection. After being given shelter, and after the misunderstanding with Beelzebub is cleared, Halkara decides to stay with Azusa and restart the production of her energy drink. She is very knowledgable about mushrooms, but scatterbrained and accident-prone.


 * Beelzebub (ベルゼブブ)
 * An about 3,000-year-old high-ranking female demon, the Agriculture Minister of the Demon Realm, and Lord of the Flies. Chasing after Halkara after getting addicted to her energy drink (so that its production could be re-started) she challenges Azusa to a fight, but collapses from exhaustion and is nursed back to health by her and Halkara. After the misunderstanding was cleared, she leaves peacefully, but vows to come back to settle her score with Azusa (which she ultimately loses) and get more of Halkara's drink, although she begins to settle in as one of Azusa's "family", eventually becoming Azusa's adoptive older sister.
 * An about 3,000-year-old high-ranking female demon, the Agriculture Minister of the Demon Realm, and Lord of the Flies. Chasing after Halkara after getting addicted to her energy drink (so that its production could be re-started) she challenges Azusa to a fight, but collapses from exhaustion and is nursed back to health by her and Halkara. After the misunderstanding was cleared, she leaves peacefully, but vows to come back to settle her score with Azusa (which she ultimately loses) and get more of Halkara's drink, although she begins to settle in as one of Azusa's "family", eventually becoming Azusa's adoptive older sister.


 * Flatorte (フラットルテ)
 * An over 400-year-old blue dragon girl who first meets Azusa and her extended family while she and her flock attempt to disrupt the wedding of Laika's older sister, out of simple jealousy that her blunt demeanor drove away her own fiance. After Azusa single-handedly thrashes the aggressors, and a final act of aggression is stalled by Beelzebub, she is forced to sign a peace treaty between the red and blue dragons. Afterwards, as part of her peace obligations, she also (much to Laika's disgust, which results in them becoming rivals) joins Azusa's troupe—the only alternative being to commit suicide due to her culture.
 * An over 400-year-old blue dragon girl who first meets Azusa and her extended family while she and her flock attempt to disrupt the wedding of Laika's older sister, out of simple jealousy that her blunt demeanor drove away her own fiance. After Azusa single-handedly thrashes the aggressors, and a final act of aggression is stalled by Beelzebub, she is forced to sign a peace treaty between the red and blue dragons. Afterwards, as part of her peace obligations, she also (much to Laika's disgust, which results in them becoming rivals) joins Azusa's troupe—the only alternative being to commit suicide due to her culture.


 * Rosalie (ロザリー)
 * A ghost girl who died about 200 years previously at the age of 15. The daughter of a merchant who had fallen on hard times, she committed suicide after learning that her father wanted to sell her off as a prostitute. Her restless ghost began haunting the merchant's estate, which later became the site for Halkara's new energy drink factory. After being caught by Azusa, Halkara and Beelzebub, she is, with some effort, relocated to Azusa's house and joins her adopted family.
 * A ghost girl who died about 200 years previously at the age of 15. The daughter of a merchant who had fallen on hard times, she committed suicide after learning that her father wanted to sell her off as a prostitute. Her restless ghost began haunting the merchant's estate, which later became the site for Halkara's new energy drink factory. After being caught by Azusa, Halkara and Beelzebub, she is, with some effort, relocated to Azusa's house and joins her adopted family.


 * Fatla (ファートラ)
 * A leviathan woman, Beelzebub's secretary, and Vania's older sister. She and Vania are both shapeshifters, able to assume the form of a gigantic, flying whale. Despite the myth that leviathans are water creatures, they are actually air dwellers, and are usually employed as living transportation (complete with a resort hotel on their backs) by demons, devils and their privileged guests. To fulfill this obligation, both sisters work in tandem: While one does the transport, the other uses the hotel facilities (especially the bath) to relax.
 * A leviathan woman, Beelzebub's secretary, and Vania's older sister. She and Vania are both shapeshifters, able to assume the form of a gigantic, flying whale. Despite the myth that leviathans are water creatures, they are actually air dwellers, and are usually employed as living transportation (complete with a resort hotel on their backs) by demons, devils and their privileged guests. To fulfill this obligation, both sisters work in tandem: While one does the transport, the other uses the hotel facilities (especially the bath) to relax.


 * Vania (ヴァーニア)
 * A leviathan woman, Beelzebub's assistant secretary, and Fatla's younger sister.
 * A leviathan woman, Beelzebub's assistant secretary, and Fatla's younger sister.


 * Pecora / Provat Pecora Allières (ペコラ / プロヴァト・ペコラ・アリエース)
 * An over 1,000-year-old demon girl and the "King" of the Demon Realm from its capital city Vanzeld. Despite her awe-inspiring title, her natural form is a cute, petite girl with ram horns and three tails, and she is, most of the time, surprisingly cheerful and informal. She has also a submission complex; after being accidentally knocked out by Halkara, and with Azusa beating her in a duel over Halkara's life, she gladly submits to her "conqueror".
 * An over 1,000-year-old demon girl and the "King" of the Demon Realm from its capital city Vanzeld. Despite her awe-inspiring title, her natural form is a cute, petite girl with ram horns and three tails, and she is, most of the time, surprisingly cheerful and informal. She has also a submission complex; after being accidentally knocked out by Halkara, and with Azusa beating her in a duel over Halkara's life, she gladly submits to her "conqueror".


 * Smart Slime (賢いスライム)
 * In manga: Smarsly / Smart Slime (賢スラ / 賢いスライム)
 * In anime: Intellie / Intelligent Slime (賢スラ / 賢いスライム)
 * A normal slime which is completely black.
 * Smarsly (in manga) / Intellie (in anime) is name given to it by Azusa. (Shortened name is used in light novel volume 12 and later.)


 * Wizly / Wizard Slime (マースラ / 魔法使いスライム)
 * In anime: Magie / Magician Slime (マースラ / 魔法使いスライム)
 * A magician slime girl who looks like 15-year-old blond girl, but is 300-year-old woman.
 * Wizly (in light novel and manga) / Magie (in anime) is name given to her by Azusa.
 * Wizly (in light novel and manga) / Magie (in anime) is name given to her by Azusa.


 * Fighsly / Fighter Slime (ブッスラー / 武道家スライム)
 * In anime: Fightie / Fighter Slime (ブッスラー / 武道家スライム)
 * An about 300-year-old fighter slime girl whom Azusa and the others seek out when Falfa is stuck in slime form after getting a crick in her neck. She is very materialistic and considers fighting a means to collect more and more prizes. After being easily beaten by Azusa, she considers joining her as a martial arts apprentice, but instead becomes Beelzebub's apprentice after learning about the slimes that Azusa killed.
 * Fighsly (in light novel and manga) / Fightie (in anime) is name which she herself decided to use, and was not given to her by Azusa.
 * Fighsly (in light novel and manga) / Fightie (in anime) is name which she herself decided to use, and was not given to her by Azusa.


 * Eno (エノ)
 * An over 150-year-old immortal witch, called "the Witch of the Grotto" (洞窟の魔女). Naive and deadly shy, but yearning for recognition, she usurps the title of "Highland Witch" to capitalize on Azusa's reputation and make herself some new friends. When Azusa hears of the fake, she confronts Eno and afterwards helps her in selling her mandragora-based medicines, which prove extremely popular. Her title stems from the location of her workshop, which is situated inside a hard-to-reach underground location, thus contributing to her initial lack of renown.
 * An over 150-year-old immortal witch, called "the Witch of the Grotto" (洞窟の魔女). Naive and deadly shy, but yearning for recognition, she usurps the title of "Highland Witch" to capitalize on Azusa's reputation and make herself some new friends. When Azusa hears of the fake, she confronts Eno and afterwards helps her in selling her mandragora-based medicines, which prove extremely popular. Her title stems from the location of her workshop, which is situated inside a hard-to-reach underground location, thus contributing to her initial lack of renown.


 * Pondeli (ポンデリ)
 * An undead catgirl who died 40 years ago after she became so lazy from boredom that she forgot to even feed herself. After rising from death, she became a graveyard security guard, but felt lonely without anyone keeping her company. Azusa solves the problem by suggesting to Beelzebub that Pondeli is made a demon casino gambler, since the catgirl is wickedly good at playing cards.


 * Leila
 * Laika's older sister, who marries later in the story.


 * Natalie
 * The current owner of Flatta's guild.


 * Kuku (クク)
 * An about 80-year-old almiraj minstrel girl. She also has an alternate persona called Schifanoia, which she later abandons.
 * An about 80-year-old almiraj minstrel girl. She also has an alternate persona called Schifanoia, which she later abandons.


 * Yufufu (ユフフ)
 * A droplet spirit woman.


 * Sandra (サンドラ)
 * An over 300-year-old mandragora girl.
 * An over 300-year-old mandragora girl.


 * Misjantie (ミスジャンティー)
 * A pine spirit woman. The being who mediated marriages.


 * Curalina (キュアリーナ)
 * An about 60,000 to 70,000-year-old jellyfish spirit and wandering artist woman.


 * Goodly Godly Godness (メガーメガ神)
 * The goddess. The very being who reincarnated Azusa into this world.
 * The goddess. The very being who reincarnated Azusa into this world.


 * Muu / Muum Muum (ムー / ムーム・ムーム)
 * A ghost girl. Sovereign of the ghost's kingdom of the dead. She died at 15 years old, and about 5,000 years have passed since her death.


 * The Goddess Nintan (ニンタン神)
 * The goddess of time. A goddess long worshipped in this world.


 * Wynona (シローナ)
 * A slime spirit girl born after Falfa and Shalsha.


 * White Slimes
 * Not much is known about them other than the fact that they are evil.


 * Wyverns
 * They are used as a faster way to travel up the World Tree.


 * Red Dragons
 * Laika's kind, who can also take the form of humans.


 * Blue Dragons
 * Flatorte's kind, who are known enemies to the red dragons until a peace treaty was formed by Azusa. Like the red dragons, they can take human forms.

Locations

 * Flatta village
 * This is the story's main setting. Azusa lives close to this village.


 * Mount Rokko
 * This volcano is the homeland of the red dragons. This is where Laika came from.


 * Demon Realm
 * A city where demons reside. This place is controlled by Pecora.


 * World Tree
 * A giant tree that has been hollowed out and turned into a tourist attraction. It has 108 floors, each containing different challenges.

Light novels
Originally, the series was published by Kisetsu Morita on the user-generated web novel site Shōsetsuka ni Narō in 2016. The publisher SB Creative acquired the series and then began publishing it on their GA Novel imprint as a light novel in January 2017. Twenty-four volumes have been released as of November 2023.

In the volumes 5 to 7, 20, and 21 of the light novel, a special story about the character Beelzebub was published at the end of those volumes. In September 2019, those stories were compiled into its own volume and the spin-off, called I Was a Bottom-Tier Bureaucrat for 1,500 Years, and the Demon King Made Me a Minister (ヒラ役人やって1500年、魔王の力で大臣にされちゃいました), published by SB Creative on the GA Novel imprint with Kisetsu Morita and Benio as the author and illustrator, respectively.

In the volumes 8 to 10 of the light novel, another spin-off light novel called Food for an Elf' (エルフのごはん), a special story about the character Halkara was published at the end of those volumes.

In the volumes 11 to 13, 16, and 18 to 19 of the light novel, another spin-off light novel called Red Dragon Women Academy (レッドドラゴン女学院) a special story about the character Laika was published at the end of those volumes.

In the volume 17 of the light novel, another spin-off light novel called The White Journey of Margrave (辺境伯の真っ白旅), a special story about the character Wynona was published at the end of those volume.

Manga
In June 2017, Square Enix began publishing a manga adaptation of the series by Yūsuke Shiba in their online website and smartphone app Gangan Online, with eleven tankōbon volumes released as of June 2022.

A manga adaptation of the spin-off light novel I Was a Bottom-Tier Bureaucrat for 1,500 Years, and the Demon King Made Me a Minister was launched on Square Enix's Gangan Online website and smartphone app, with Meishi Murakami reprising his work.

A manga adaptation of the spin-off light novel Red Dragon Women Academy was launched on Square Enix's Manga UP! smartphone app, with Hitsujibako reprising his work.

Anime
An anime television series adaptation was announced during a livestream for the "GA Fes 2019" event on October 19, 2019. The series is animated by Revoroot and directed by Nobukage Kimura, with Tatsuya Takahashi handling series' composition, Keisuke Goto designing the characters, and Keiji Inai composing the series' music. The series aired from April 10 to June 26, 2021, on AT-X, Tokyo MX, and BS11. Aoi Yūki performed the opening theme song "Gudafuwa Everyday" (ぐだふわエブリデー), while Azumi Waki performed the ending theme song "Viewtiful Days!". Crunchyroll licensed the series outside of Asia. In Southeast Asia and South Asia, Muse Communication licensed the series and will stream it on iQIYI, Bilibili, CATCHPLAY+, and other platforms. The company also has licensed the anime to Animax Asia for TV broadcasts.

In January 2022, it was announced that a second season was green-lit. The season will be animated by Teddy and directed by Kunihisa Sugishima, with Naohiro Fukushima handling series' composition, Hikaru Kodama designing the characters, and Keiji Inai returning to compose the series' music. It is set to premiere in 2025.

Reception
The light novel ranked ninth in 2019 in Takarajimasha's annual light novel guide book Kono Light Novel ga Sugoi! in the tankōbon category.