Against The Necrotic Death Lords!
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Kireina had been given the simple task by Mammon to slay these invaders. To start off their alliance, she wanted to see what Kireina was truly capable of, sending her to war and fight off these thugs was a nice way for Kireina to get some nice and exotic snacks while also fending off this pest from the Gates of Hell.
The four Necrotic Death Lords were caught red-handed as they were absorbing the demonic energy through their grotesque Undead Formation and were also forming new demonic cores. Kireina didn't thought it a single second. She quickly infused countless skills, techniques and spells into her body and Divine Aura, expanding it widely as she teleported right next to the Undead Formation!
Rimuru and the rest of her family and allies also were teleported with her, as they surrounded the Necrotic Death Lords with their powerful auras!