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The Necrotic Death Lords had infiltered the Hell Gate a few days ago, just before Kireina entered it. however, unlike her, they entered through the actual gates and were randomly sent to the Greed Layer of Hell, where they met with Plutus, who was sleeping after drinking alcohol.
The Necrotic Death Lords noticed the level of power he had, and they quickly used this opportunity to run away from him while he slept, infiltering deep into the entire layer and investigating its interior in detail.
The skull-faced entities whose power was as high as Supreme Gods if not higher had been suppressed due to the power of Hell. Even with this Archdemon Avatar Undead as their anchor, it didn't meant they would be able to freely traverse everywhere.
Check "The Protagonist's Sister Is Actually the Strongest"!