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While Flora, Aura, and the System Master fought against the Necrotic Plane Ruler, at the same time, Rimuru and the rest of Kireina's family, alongside Loki and her two children confronted a new foe!
Or well, the same foe that Flora and the others were fighting, the Necrotic Plane Ruler made himself present once more, as he stopped Loki from getting through his brainwashing of Hel.
And how? Well, by merely possessing her body completely!
After all, Hel was merely his new vessel, a compatible incarnation he wanted to use to conquer Genesis. She wasn't someone he even cared for at all.
In fact, the Necrotic Plane Ruler was an eldritch, outer being born from Death and the Void, and held no emotions other than greed and the desire to become stronger to dominate and conquer more.
Check "The Protagonist's Sister Is Actually the Strongest"!