Okay, to answer a few questions:
There was no "fine print". The contracts were verbal, and sealed with a handshake, which seared his mark into their flesh. The scar can't be removed.
For someone who owes part of their soul to him, they spend one day in ten in his service, through all of eternity, for every 10% they owe. But that's only after death. In practice, it means that if they die there is no way to bring them back, since he'd have to let them take the portion he owes.
Why is it to be a combat encounter? Because the PCs are planning to attack him. That's not a condition I get to change. They want to kill him.
They understand that he'll return to Hell as a Llarva, the lowest form of Devil. From there, if he isn't eaten by one of the big guys, he'll slowly have to work his way up again. But any contracts he holds stay with him, and he can trade them to accelerate that process.
For that reason they're looking for a way to not just kill him, but to utterly destroy him, body and soul. That's a lot harder.
As for binding the PCs to him, he has no way to force it, but he has leverage. One owes him a service. He will make three offers, which the PC can refuse. If he refuses them all, then his debt goes from 50% to 100%. That one's already been offered one such service, and refused. If the Devil offers a second one, and has it refused, then he can make the third at the start of the battle. If the PC refuses, the Devil owns him completely.
The other PC is in the process of completing a service. But he's decided to fall on his sword, so to speak, by disposing of the prize so the service can't be completed. He will own that PC completely when he fails to pay.
Problem: They're planning the Devil's demise before zero hour, so he won't get to play that card.
As for class levels: I could see adding Monster dice. I could see adding class levels. Mixing both just comes off strange. It's not that I can't, I just don't think it will work well, and I can't add enough to make a difference in this fight. Push him to CR 20 with straight levels and he'll be 60% of one of the PCs, and he'll still get smeared, and fast. So I need to try something else.
I like the Monster of Legend idea. I'll have to look into that one.
