[Agema]

Horror marking is best done with massed targetting on important units, like thugs, mages and other commanders.

Hence astral geyser is not a great choice. It's got mediocre accuracy so you're not likely to hit the right target, and in my experience it targets groups of basic troops anyway. It's mostly useful for the 2AN,MR area damage: although it doesn't do much damage, the range is good and fatigue cost low. If you have blood as well, you could fire astral geysers and then summon horrors, which will preferentially attack the horror marked units.