In theory, a horror mark gives a certain unknown chance to summon a horror in a turn. This Horror will then, Assasination like, attack. This Horror will also make the Horrormark worse, because it has Horrormarking attacks. Eventually, ****ing scary horror will turn up, that should have some good odds at even killing an SC.
I really hope for this guy to show up:
http://dom3.servegame.com/wiki/Eater_of_Gods
Being Horrormarked will also cause Horrors Summoned in battle to prioritize the horrormarked dude.