CrazyLoon said:
Yes, with the green sphere and all the memories of his previous incarnations, and supposedly absorbing all of the Transcendent One's energy, would make him absurdly powerful. However, the fact of the matter is, he is no longer an immortal and the Blood War is ended. His only reasonable fate would be death during battle. If he has gotten out alive, it would make his punishment sentence a complete joke.
His punishment is simply the fact that he is stuck there. There are demons who are damn near godhood, and several that are more powerful that most gods. And they still keep trying to get out into even the pissy old primal plain, because anything is better than the 9 hells.
The punishment is being trapped there, not the threat of some behemoth killing him (though upon joining the blood war, he'd instantly be the centre of some very unwanted attention - both to use him, and by those seeking to kill him before he becomes a bigger threat. Being ambushed by Demogorgon and his pals (i.e. army) in Demogorgon's home realm, is likely to cause serious trouble for TNO, even by that point of the game.
Keep in mind - TNO is godlike - but he can be swatted like a fly by the Lady of Pain, would seriously struggle against a Tarrasque (actually he'd probably get wtfpwned by the Tarrasque), and most (maybe not all, but most) gods would still be capable of killing him without too much challenge. Demogorgon and a handful of others are more powerful than most gods, so long as they are in their 'home territory' so to speak, as opposed to their mere 'avatar' which is what appears when they get summoned.
Graz'zt, Orcus, Dagon...they're all considerably bigger fish than the TNO insofar as the abyss and hells are concerned. TNO would start out as a mid-level to big player, and might truly become a major player in the underworld if things go well, but he sure as fuck wouldn't be there yet - beings with godlike power aren't all THAT rare down there, and there's plenty of folk with vested agendas on making sure he never becomes a major player.
Actually, considering that his MOST powerful form (merge with all incarnations, merge with TTO) implies a goodness in him, there's an excellent chance that Graz'zt, Demogorgon, Orcus, Dagon etc would put their differences aside for the moment and crush him ASAP, rather than let a powerful and good-aligned fellow run around unmolested. Expect him to then be kept on a very very short leash, with some form of magical control device to make sure that he isn't going to be his own man any time soon - assuming they don't outright annihilate him.