Well, drop your dirndls and unstring my lute—we’ve reached the letter I! (What, you were looking for the hyena? We covered that all the way back in December.)
The iathavos is less a race of qlippoth (3.5 players, read that as “obyrith”) than it is the qlippoth—an expression of the breed so singular the universe only allows one to exist at a time. (There are stronger qlippoth lords out there, but achieving that status requires a personality, will, and sense of self (not to mention mortal worship) that necessarily pollutes them with enough mortal/demonic essence to make them more demon than qlippoth.) Killing or incapacitating the CR 20 iathavos is a campaign-ending feat that’s as likely to see the PCs teamed up with demon lords and archdevils as it is angels and the gods of good. This is a creature that can destroy you with a 40d6 disintegrate, engulf you and rebirth you as a smaller qlippoth, or dissolve you with the acid of its own tentacular ichor. Oh, and it holds grudges—against entire worlds. Approach with caution.
Enforcers of the church are sent to stop the rise of a nascent demon lord before he reaches apotheosis. But they are soon out of their comfort zone, as reaching the demon in his home requires bargaining with devils and crewing an airship captained by a treant (who is also the ship’s rudder) with multiple personalities. When they arrive in the Abyss, they find that the demon is already under attack by the iathavos—and while their quarry proposes a truce, the iathavos is in no hurry to see them go…
A party of plane-hopping adventurers stumbles upon a night-shrouded world caught in a battle between the planet’s vampire liege lords and the iathavos and its armies of nyogoth spawn. They escape, but not without rousing the interest and ire (essentially the same emotion) of the iathavos, who endeavors to follow them to their home world and wreak destruction.
An iathavos of immense power has just been slain—at the cost of two solars, an empyreal lord, a totem spirit known as Cousin Mosquito, and the demon lord of diseases of the blood. Now a new iathavos is about to be born. The forces of good hope to kill the young, (comparatively) weak qlippoth, entrapping it so that it is removed from the multiversal playing field without the Abyss spawning a new incarnation.
—Pathfinder Bestiary 2 222–223
Admittedly, the iathavos illustration in the Bestiary 2 doesn’t quite capture the creature’s essence. But the stat block does.