At 19th level (or maybe 20th if you want to be very capstoney), maybe start nullifying epic spells?
Nullify Ability overlaps with Nullify Spell (maneuvers, at least, are explicitly extraordinary and/or supernatural).
Is Voidstrike supposed to be a standard action to use or in place of an attack? As written, it doesn't actually say it's in place of an attack (just that it's a melee touch attack), so it defaults to a standard action, which at 1d4/level kinda sucks. Improved Voidstrike's reference to damaging multiple times in a round implies that it's in place of an attack.
Will of the Void: Not quite Divine Grace, but close enough. Hokely dokely.
Touch of the Void: If you use extra immediate actions, how many swift actions do you use up form your next turn?
The class still feels like it's missing something (did you see what I did there, because the class is all about nothing and nothingness and... I'll be quiet now). You can say "no" a lot and in many ways (well, actually, it's pretty much the same way over and over, but it's contextually different-ish). But isn't one of the tenets of good DMing saying "yes" instead? I think that's true for character classes and homebrewing, too. Or, in other words, give this sucker something to actively do, rather than reactively undo.