Just a short warning to DMs planning to throw a lot of incorporeal spawn-creating undead into their campaign. A worrying feature of these creatures is that if left unrestrained and allowed to invade a few humanoid settlements, there’s a very high chance they’ll not just kill everyone in the city, they’ll raise them as spawn, multiplying their numbers exponentially until even a twentieth-level player character can’t beat them all. Think I’m worrying too much? It’s happened to me twice!
It’s like a katamari. Lets suppose a lich creates a shadow and orders him to kill everyone in a village. The shadow kills a farmer in his sleep – any given shadow automatically beats a farmer without resistance – and the farmer rises as a shadow. The shadow’s side has now twice the numbers. The two shadows kill another two farmers, creating two more spawn, until the village is empty and the shadow army numbers one hundred. Nine villages later, it numbers a thousand – easily enough to overwhelm a city. Killing is easy for them, and killing makes them stronger.
The most dangerous thing is perhaps that high numbers of incorporeal undead continue to be a threat even at high levels. In one such event a player character controlled wraith managed to amass an army of a hundred wraiths, who ended up coming under the control of an NPC and attacking the player characters. Since incorporeal creatures can essentially fly, they can surround a creature in a kind of cubic flank, in other words my poor 20th level dwarf fighter took twenty-six attacks per round each draining 1d6 Constitution on a successful hit. A CR5 creature ought not to be a challenge to him, but +7 is still powerful as an incorporeal touch attack, ignoring all bonuses but Size, Deflection and Dexterity, and bypassing his hitpoints entirely. In short, they’re a potential weapon of mass destruction and even Drizzt would be utterly screwed.
If you’re planning on DMing with undead who can create spawn and are the kind of DM who enjoys consistency in his game world, take care to consider the reasons why incorporeal undead in the world haven’t somehow overwhelmed it. Why hasn’t some madman attempted an incorporeal genocide yet – or if he has, how was it defeated? What forces are at work in the world to prevent intelligent incorporeal undead from going on a rampage against the living, or what motivations do the undead have that prevent them from wanting to do so?