Zariel convinced an entire army to invade Hell before, if the PCs are smart, they might be able to get advice, equipment and maybe even help from a variety of places for a round 2 - ranging from the armies of good seeing this as a way to finally take Hell down for once and for all, to demonic or daemon help for their own purposes. Heck, throw a Slaad spawning stone down into Hell and see what happens - it'll be a distraction at least. Imagine should the PCs show up with an army of angels, modrons, demons, hags and daemons all chomping at the bit to put down the leaders of Hell for once and for all...Reminds me of this classic as the Knights seemed to think sucker-punching gawds was a good idea.
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But, on point, what would the other gawds think if some mortals went around with ambitions to snipe them? He's not just a big bag of hit points waiting to be snuffed. Each of the gawds in the Realms serves a purpose, for good or ill, in keeping the universe running.
@tglassySo now we're playing a game where the primary goal will be to go through each of the nine hells until they can ultimately face the statless Asmodeus. Now, I'm well aware of who Asmodeus is. I know he's essentially a Greater Deity with absolute control over the nine hells.
Also, put in some sort of no suicide clause. Otherwise Asmodeus would just have offed himself eons ago and left Hell to be more powerful.I like this direction - but with an add on. Not all the stronger residents want Asmodeus in place. Quite a few will be of the "better to rule in hell..." Philosophy and will be only to0 glad to replace him.
As such this is a huge opportunity for serious intrigue - which seems helped by the fact that 3 of the 4 PCs likely have extremely good Charisma (they're charisma casters) and could thrive in that kind of environment. I'm not talking about intrigue within the party (unless they're into that sort of thing) but more that navigating the Hells can be much more about political force, alliances, etc then pure physical force.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.