Not good...
The claw of just that toe had grown so long that it had curled back into the pad and there was a big dent and a lot of crusty puss. He let us clean it off, then Demo clipped the nail just above where it was against the pad. Merlin freaked a bit, and I had to adjust how I was holding him. The point of the nail came out of the pad and puss started coming out. There was a "bubble" that is bigger than the pad itself on the side of his toe, and he must have been really releaved to have it open and the pressure relieved because he let me push on it and it started oozing / globbing.
That's where I started checking out, and we had to take a break from Merlin to get me off the floor.

The claw is still REALLY long, and we only got enough off to get it out of his pad. It's got to be cut way back to get it to a length where he can take care of it by sharpening it like normal. I have him in a crate with newspaper for litter so that hopefully it will stay clean until we can get him to the vet. I don't want him using the regular litter box now that it's open and draining.
It's really nasty. I know that their claws grow continually, but I've never seen anything like this before. Has anyone else? All of his other claws are sharp and normal. Are cat quicks like dog's and it will be far up into the claw or can I clip it back and not have to worry about hurting him even more? My cats are not nasty, he'll let me do it, though if I quick him I'm sure I'll get tagged (not that I'm blaming him for that!)
Hopefully we can get him in tomorrow because he is howling his head off in the crate.
Any ideas on how to keep him comfortable or help things until he gets in?