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[Your party has accomplished the requirements to summon the Hidden Boss]:
[Do not let the Dracolich transform into a Phantom Dragon.]
[Kill the Dracolich before it transforms.]
[Leave the Dracolich's core intact.]
[The Floor 5 Hidden Boss: [Tentacled Grieving Nether Soul Fragment of Outer Death: Yer-bogarr (Star Eater Realm: Tier 5)] has emerged!]
[Yer-bogarr screams in agony over its own existence! Its voice makes anybody that listens to it have their Stats reduced by 20%!]
The grieving thing was the hidden boss of this little Cosmic Dungeon, whose difficulty wasn't higher than World Realm Tier 5! This monster, however, was on another whole Realm altogether.
It was slightly worrisome to let my kids fight this thing, they weren't as strong after all, and were just getting used to their cosmic powers. They did leveled up a whole lot though, so maybe they weren't as weak as I thought now.