[Taqwus]

The insanity is only late-era, based on dominion strength... and possibly MR, astral magic, or both. I've noticed that my Starspawn mages and priests rarely go mad; my mundane Starspawn commanders occasionally do; and the cultists frequently do. Void creatures like the Void Lurker and Void Lord are immune, or should be.

'Go mad' is actually two-stage. It increases an insanity score -- probably when some check fails. That score presumably affects the probability of a commander going insane each turn -- they can shift between lucid and random from turn to turn. Insanity also accumulates for enemies within your domininon.

If you get a fellow who returns from the void, and lives long enough to return to the capital, check his summoning score.