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while rimuru is delayed by hinata, the church lends an army of 10-15,000 to the falmas kingdom and they invade tempest and massacre about 100 citizens of tempest, including shion, which makes rimuru flip out in rage and massacre the entire army. this fulfills a requirement for evolution into a true demon lord. and thus demon lord rimuru is born (≧∀≦)

Another spoiler ........................................................................................................................................................................................................................................................................................................................ Shion and the others do not stay dead since Rimuru went nuts on the army in order to save their lives.

https://www.readlightnovel.org/tensei-shitara-slime-datta-ken-wn/chapter-239
Its a webnovel but its kinda similar hahaha

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Stone cold dead but rimuru got an ultimate skill that can rewrote everything basically cause it knows everything. Wisdom king raphael its one of the 7 virtue series of the original creator of the world veldanava aka milims father. Welp if you want to talk about it I can talk about it more. Chloe is a much interesting kid than you think. Hehe. Try reading the other stories, there are baron demon diablo and his arch dukes demons. My personal favorite is kumara.

Milim just became a demon lord because someone killed her pet dragon and she lost it and killed thousands. She is actually the kin of the original creator of the world. Diablo on the other hand is one of the stronger creations. Afaik, angels lack free will and demons want the will to live. Something like that, in anime you can see diablo saying rimuru will show dewae or something because its their purpose in life.
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Cant wait for the unholy massacre. (=・ω・=)