Finally, the monster that is responsible for my crappy dice rolls.
The Catastrophant is most definitely a boss monster and an interesting one at that. I can say for certain that it looks like an eldritch abomination; the art here is high quality, the design is excellent, and all the information is easy to follow. The writing is solid, and gives us info about its modus operandi. As befitting what will most likely be a major big bad, the Catastrophant has a set of interesting mechanics. Its major gimmick is that it gets stronger the more fumbles occur in its presence. Of course the more someone fumbles the more likely they are to detect the Catastophant. This is an interesting mechanic and makes the Catastrophant extremely deadly, as befitting a boss in Mork Borg, though a character needs to fumble in the first place. Of course, that's why you have other monsters. It might be worth having the Catastrophant to engineer a few 'incidents', and use them to give players a hint that something is definitely a foot; that barrel that fell and nearly crushed the seer seemed to have been secure, someone most of cut a rope.
While its main mechanic is definitely the highlight, I will say that some players may also find it frustrating. Deathspirials can become difficult to deal with, especially those that are caused mostly by bad luck and can't be manage easily. Its for this reason I would advise against using the Fate Taker ability often unless a player has got a lot of omens stockpiled. As a counter, I will also say that this is Mork Borg, so most players won't mind losing their character are they fall on their own sword (literally); this means my concerns are ultimately for nought but I'll still mention them.
Overall, its a Catastrophant is a good example of a Mork Borg boss; it has a design that is horrifying, and an interesting set of mechanics that make it challenging to fight while not being overly complicated.