Haven’t played WG4, but I used Tharizdun as the primary evil force in a homebrew 5e game a few years ago. His titles are very evocative, but he is also amorphous enough to do a lot of creative things with. You can go the elemental route, but I’m partial to “Chained Oblivion” and “Black Sun.” Also, a party might be curious if a cult is trying to release Tiamat or some other incredible god, but it is very easy to understand that Tharizdun would simply end existence. All this also contributes to a lot of possible minions, cultists, high priests, etc, that fit different roles.

The one issue I actually had was that my PCs took the threat of Tharizdun so seriously that they kinda stopped getting wrapped up in side quests. Not the worst thing, but it might be a good idea to keep him as a figure obscured until you can really put the pedal to the metal.