The result of a sliver of Yog-Sothoth being injected into reality. Every iteration is unique.
In Chapter 178, the party encountered one inside the Mysterium: a squirming, crusty stained mess of sludge and scar tissue almost the size of an elephant, continuously producing tentacles, heads, eyes, elephantine feet. It existed naturally invisible. Bite plus four tentacle attacks with reach and grab, plus blood drain. Spell resistance.
First Reference: Chapter 175
Other Notable References: Act IV Interlude Part 5, Chapter 178, Chapter 183