Re: Mad Cult-Leader of the Elder God of Pestilence
Definitely Sticky powers to represent contagion. Possibly with Invisible Power Effects so that the contagion can spread (with Delayed Effect). AOE attacks, in general, would be fitting.
Also, depending on how you want the priest to look, he could have powers that stem from his own body having been given over to corruption. Regeneration (ironically), Damage Reduction for being goopy, maybe even Stretching or Desolid... I'm kind of picturing a priest that goes around mostly shrouded since only his head and face do not appear completely gangrenous, then whips the gloves off his decaying hands and arms when it's time to throw down.
You could google Nurgle, too, and see what comes up.