I don't mean the Ever-Lovin Blue-Eyed Thing of the Fantastic Four. I mean the Thing , also the Thing from another world, or the "Who" in "Who Goes There?" The shapeshifting alien baddie from the classic 80s John Carpenter film, that mimics anything it assimilates, assimilates everything it comes across, regenerates, shapeshifts, splits itself into multiple pieces if need be, can assume giant nasty combat forms if pressed, and is generally unpleasant.
I've used it in a game before, and I kind of want to follow up on the idea, but that was back in OCR/RCR, so I really have no idea what to do with it in FFG terms.
Key points:
It's difficult to hurt. Each individual cell is basically its own living organism, so Things don't have vital organs that can be damaged to shutdown. The only reliable way to kill it is to burn it sufficiently that all the cells die (acids and other things that would destroy large amounts of cells easily would probably also do the trick).
Related to the above, it's what TvTropes calls an "Asteroids Monster." Cut it in half, you have two smaller ones, each wholly independent and capable of ruining your day.
It assimilates other beings, infecting them like a virus, consuming or transforming their cells into its own (the PCs and significant NPCs for the adventure aren't going to get infected. . . but they don't know that, evil laugh).
As a result, it can split parts of itself off from the whole, to try and escape and spread or attack from multiple vectors, or can merge multiples of itself into larger colonies to create large and powerful monsters for the party to fight.
So. . . any ideas on how to approach this?