That Time I Reincarnated as a Slime was lauded after season one for breaking away from the typical foibles of the fantasy isekai genre and dishing out a fun and engaging anime for the audience. The Isekai genre is plagued with grim undertones which are further dragged down by a bored protagonist.
In contrast, this anime is a refreshing and positive show built on good writing. The anime is set to launch season two in 2020 about two years after its debut. The first season left us with a lot of questions and the most compelling one is -- “When and How Will Rimuru Become a Demon Lord?”
Note: As mentioned above, Rimuru does not become a Demon Lord in the anime, i.e., the first season. Hence, this article from here on is purely based on spoilers from the light novel.
1. Quick Answer
Rimuru becomes the Demon Lord in the Birth of the Demon Lord Arc in the Web Novel/Light Novel. This happens after he sacrifices 10,000 enemies to revive his friends by becoming a Demon Lord.
2. Long Answer
When does Rimuru become a Demon Lord?
In the Web Novel, the Birth of the Demon Lord Arc is the fourth arc in the story. It takes place after the Life in the Royal City arc. By the end of its first season, the anime has managed to cover the Life in the Royal City arc.
Therefore, Rimuru will definitely become a Demon Lord in the upcoming second season of the anime. As season two is likely to start with the Birth of the Demon Lord Arc, it will divert from the usual comedic tones of the show to a darker one -- as it covers the attack on Tempest and deals with Rimuru’s reaction to it.
The anime does not cover the events of the web novel holistically and deviates from it quite notably. Therefore, one cannot say with conviction what elements from the web novel will make it to its audio-visual adaptation.
How does he become a Demon Lord?
As per the web novel, soon after Rimuru saves the souls of the five students under Grandmaster Kagurazaka Yuki, he receives the news of an attack on Tempest.
On his way to Tempest, Rimuru is interrupted by Hinata Sakaguchi, the person shown tailing for the last few episodes of the season. He is forced to battle her but soon retreats. Tempest is besieged by the people from the Kingdom of Farmus -- ruled by King Edomalis. The attack is conducted by the Blood Shadows -- an organization under the leadership of Hinata Sakaguchi.
This attack on Tempest resulted in multiple casualties, that of Shion included. With Rimuru enraged and grieving, the Great Sage provides him with a solution. To resurrect the fallen Tempest citizens, Rimuru must become a True Demon Lord. That being said, Rimuru must sacrifice 10,000 souls to gain that power and ascend the title of a True Demon Lord.
Motivated by his seething rage, Rimuru seeks vengeance and obliterates the invading forces of the Farmus military. He further summons an Arch Demon and his two Greater Demons to hunt down the only survivor, Edomalis.
Rimuru can now revive the fallen citizens of Tempest (Shion included) and can give “gifts” such as species evolution and skills through the Soul Corridor he shares with them.
Note: In the Light Novel, the attack was conducted by the OtherWorlders -- bound as living weapons to the country of Farmus. The Blood Shadows never appeared or mentioned.
Inconsistency of Medium’s Adaptation
That Time I Got Reincarnated as a Slime was first published online as a Web Novel in 2013. It was later published as a Light Novel in 2014. Further ahead, the story was adapted into a manga form in 2015, and then its anime debuted in 2018.
Long-time fans or rightfully the manga/light novel fans are aware of the inconsistencies to the plot between the manga and the light novel.
As asserted by these fans, the Web Novel is a rough outline of the plot that was fleshed out in the Light Novel. The manga form was created later yet it varied from its predecessor in minor ways. With the first season completed, they assert that the anime was heavily reliant on the manga more than the Light Novel. So, the anime may not do justice to the light novel fans, but it does attempt to adapt the manga as best as possible.
Therefore, one can speculate that the events of how Rimuru becomes a Demon Lord may or may not be faithfully adapted from the light novel/web novel. There is no confirmation yet as to what one can expect from season two.
3. Plot of TenSura
Satoru Mikami is unfortunately stabbed to death while accompanying his colleague and his partner. Upon reflecting on his miserable life, he is filled with immense regret. He then makes certain requests during his death. Soon, he finds himself reincarnated into a different world as a predator slime that can absorb or devour anything and obtain their abilities. In this strange world, Mikami dons the name “Rimuru Tempest” and befriends a dragon Veldora. Rimuru ventures into the world trying to help Veldora unlock his seal. The story focuses on his acclimatization to this new life and the fantastical challenges it brings.
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