[Endoperez]

There's no way to see it, but Horror Marks vary in power. One Horror Mark cast at your pretender will send any horrors called to the battle against him, but won't call in any horrors. Or if it does, it's one of the Lesser Horrors which can die to one well-placed spell or one hit from a magic weapon. There's an Evocation spell, Astral Geyser, which causes more severe horror-marking, and takes Astral 3 IIRC.

You have every right not to like the Horror Mark spell, though, and if you make a mod of your own, put this line in:

#modname "test"

That's enough to make the mod work, and you can add any other commands into it afterwards. New mods only affect games created with the new mod active; changing mod on the fly makes the changes more or less immediately. It might take a turn until e.g. the research level of a spell changes, but most changes are instant.