Like Hastur's Gaze, potentially game-ending if the stars align. If this ends up in your hand and there are no Monster enemies left set aside, it'll stick around forever, and prevent you from triggering abilities on locations... like, say, the Resign ability on Garden added by No Asylum. Probably unintentional, but still devastating, especially given the "Investigator Defeat" resolution nastiness.

Aside from that, surprisingly gentle; yes, not being able to help the party complete the 4 tasks needed to advance Act 3 is troublesome, but there are still things you can do with your turn, even if you never discard this (until you need to resign). For example, you could kill The Man in the Pallid Mask, who really seems to like spawning in Garden, preventing you from being able to resign until he's exhausted...