What is the nature of this hell, and of the likelihood that it exists?
The likelihood that it exists is zero, so no need to worry about the nature of it.
I mean, in any situation where we’re talking about receiving infinite money, I don’t think real-world logic applies.
In particular, I would think that any entity with the capacity to offer this deal would know more about the likelihood that an afterlife exists than any of us do or would.
And there's one supernatural entity in particular who's known to offer exactly these kinds of deals...