[Taqwus]

R'lyeh has access to astral and water, letting even mundane commanders do some damage with Herald Lances (faster, with boots of quickness). Commander-killing rituals like Mind Hunt, or the ever-funky Dreams of R'lyeh (which doesn't have the feeblemind risk that Mind Hunt does, IIRC) are also accessible.

Some void summons will mind-blast; some others are good at stomping (Greater Othernesses are tramplers, for instance). And void summons are sacred, if you care for a bless. A Vastness not only has multiple mindblast attacks per turn, it also hits back at the mindless hordes via blood vengeance...

And if you last long enough, your dominion will spawn you many H3 priests due to insane cultists declaring themselves prophets.