The Hidden Castle of Yob in the Miznia Marshlands was raided by Bozbik the Face Breaker during the Great Monster Uprising. Before finishing his conquest, something happened within the castle the caused a tentacled monstrosity to slowly push its tentacles into Zork from another plane of reality. Bozbik was the only adventurer who learned about it in time to forever seal it back into whatever horrific eldritch dimension beyond the understanding of feeble mortal minds from whence it came before it could have spread faster than grues from a pit.

SOURCE(S): Legends of Zork (game, personal correspondence with author of 'Enter the Face Breaker' quest)