The idea is the following: Pythium´s arch-theurgs come out of the box with 3 astral, and often have other nice things along with that. You script one (or more) to cast horror mark several times, then Return (make sure they have two pearls). Then you teleport the Doom Pope into some province where you know there is some unit which could be improved by horror marking (particularily pretenders). With some luck you´ll both horrormark the sob, and manage to teleport home. Plus, I dont think there is any reliable defense against this barring keeping a stack of flying units to kill the Doom Popes upon arrival. Missile units will work against out of the box Doom Popes, but if you make them wear Shadow Cloaks, or air shield amulets, that becomes almost irrelevant.
A possible variant: spam Astral Geyser instead of Horror mark. Use multiple Popes. I still have to try this one, but I think that with a big enough number of popes you could overwhelm PD.