[Twan]

If I've understood correctly (and my memories of using this spell had results around this) rain of stone has one effect per square and so touches about 1/3 of all units per cast (if they are size 2, and so are 3/square).

Like earthquake, it's mundane dammage, so body ethearal works against this, and mistform is only negated if dammage roll is high (but it's less safe than earthquake, the 14 dammage + DRN often remove mistform on some protection 0 units like unfortunate mages).