Underworld Guardian is an NPC in Niflheim (@jump 315 55).

If you've joined The Chaos, Wish Maiden will send you to Petra, who'll tell you to find the Guardian of the Underworld who can grant you approval to access Hell.

After talking to Petra, this message should be broadcasted:

"Petra: The force of darkness is growing in numbers!"
Only after that will the Underworld Guardian grant you approval.

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