[Agema]

You probably need about 50+ horror marks from horror mark/astral geyser to get decent horrors to turn up and attack with reasonable frequency. Depends to an extent on the SC you're facing too: one that can't hold a weapon or cast a suitable spell (eg. gifts from heaven) can be in real trouble as they have severe difficulty killing the horror.

Astral Geyser can still be a useful spell, but for the damage not the horror marking. It has got good range and decent AoE, and is cheap in fatigue. It is only 2AN MR, but that still means against most opponents you'll affect every second unit for an average of about 3.5 damage.