I went back through my notes, and the reference is earlier in this thread where the Cradle of Charybdis is mentioned by Gary 'past the sea of ghosts', so likely in the Sea of Monsters. I must have added the Scylla detail, because there's otherwise no reference - my bad.
Mike is a crazy good artist and he's put a lot of thought into this art. Given the cavelike nature of The Sunkiller, I've tried pairing it up with other art and I'm having misgivings that it is in Book 6, and instead in Book 5. But - Gary's synopsis that he released on Kickstarter makes me think that maybe the Sunkiller is
Take care of yourself first, Kevin - this has been sitting since 2012 and it can wait while you can't. Better yet, I think you could convince people that you could do your own adventure path. You obviously have the chips for it. But get well first.