...I think an 11 doom track ancient one with no magical resistances and a -4 check might be too easy (especially in a 3-4 investigator game). If it turns out a little too easy, change his attack so he just devours the first player each round (like Quachil, but without the double immunities). Tastes for difficulty differ though (personally, I like very tough final combat— I hate final combat with most of the base game AOs).
One thing you'll need to clarify is how the stalker movement works. Do you intend for monsters that are normally stationary to move? If not, change it to "Monsters that move also move as stalkers."
While that attack change would certainly make him more challenging, I think it would lose the flavor of what Giygas is, so I think I would like to keep the ally destroying attack. so in order to up the difficulty, do you think it would benefit more from magical resistance or a nastier combat modifier?
I intended stationary monsters to move with Devil's Machine, so I don't think I need to include an extra line to clarify. I think it helped the threat of stationary monsters by quite a bit.
Grudunza, thanks so much for the grammatical tip there. As you said, it's pretty obvious what the intention of the mechanic was, but dispelling any possible confusion is excellent. Regarding how many allies are consumed each turn, I waffled for a bit over whether it should be one ally, or all of them. Realistically, an investigator could at most eke out two allies, right? Although Ashcan Pete, or some others who start with an ally could throw it for a loop.
Thank you so much for the criticism so far, it makes me feel so warm and fuzzy. I think I'm going to start on a herald to work with Giygas. I'm going to try to build it to address the weaknesses of Giygas, primarily the terror level. I played another game with Giygas (and ended in a sealing victory again, sheesh, I want to test the final battle already.) and once again the monsters swarming the players resulted in a very controlled terror level. I'm hoping to build the herald to address that failure of Giygas, and I would love any ideas, or further revisions to Giygas himself.