BBEG Construction help

[Loremaster Howlin]

So for the final Enemy of the campaign i'm running, i wanted to have the PCs fight a representative of what is equivalent to Rovagug in this universe. I am thinking of using a extremely high level cleric or other divine magic user under it's control.

However i don't quite know how best to construct a "Uncreation-Pope" and would like some suggestions.

I'm thinking a 20th level half-fiend cleric whose domains are void and destruction, backed up by a dozen or so of the Uncreator's "children"

However, i'm worried this wont'll be effective in the least, so any suggestions for this?

[dharkus]

if you're going with extremely high lvl you might want to include a mythic tier aswell for more power
what creatures were you thinking for the children? high lvl clerics are always powerful if you pick the right spells and pre-buff

[Generic Villain]

Don't think a 20th-level half-fiend cleric would be effective in the least? Good lord man, I'm not sure how much more effective you could get without just throwing the tarrasque at them. Maybe a few great wyrms for good measure.