Cleon might have the right idea, but I don't like making the spirit unkillable. Make them a corporeal outsider that's killable by violence. Give them an SQ that dismissal or dispel evil kills them, but they have rejuvenation that's beaten by a Spellcraft/Know: religion/ Caster Level check against a DC based on the HD?
Sounds like two votes for corporeal (unknown type) and two for incorporeal outsider. Or am I perhaps misinterpreting Cleon and demiurge?
The difference for me is that I don't usually think of the animating power of undead as being demons. Devourer's, for example, are animated by a need to find their lost souls (and negative energy, of course), per the Dragon ecology. These things, well, I'm not so sure. I think we're still split on this. But I do insist on removing the demon references if we go the undead route.
Picking a number out of a hat, let's say Str 20.
Well I'm used to think of D&D undead as being created or occupied by demonic entities, since AD&D strongly implied that's how many Chaotic Evil undead came about.
That suits me. It should certainly be higher than a Spawn's.
That would give us Str 20, Dex 20, Con -, Int 10, Wis 13, Cha 19.
I'm thinking the Dex and Cha may be a tad high, we could knock off a couple of points from each to make it Str 20, Dex 18, Con -, Int 10, Wis 13, Cha 17.
Those stats work for the lessers. We'll want to boost Dex and Cha back up for the greaters.