i vote for a Prince of Death expanded into at least one other path of magic, either nature or astral. Here is why.
1) You can expand almost right away using it. And from my experience it takes a couple turns longer to really start expanding with a nation that relies on production. It needs that boost from the pretender.
2) After you have a fast enough expansion using knights, you can use the pretender as a site searcher, to expand into the addiontal paths.
3) In order to best make use of ulms forging abiliity, you need chasis' to to put the items on. Death provides the best summons for that. In particular with ulms heavy earth forging, you can put heavy encumbrance items on undead. Earth forging isn't as good for thugs that get fatigued.
4)Elephants don't like princes of death.
The only problem with this is a prince of death is very unthemetic for ulm. Ulm breaks away from Ermor to follow an anti magic, anti sacred, anti everything, cult, and then worships a prince of death?