[Taqwus]

There's also 'Horror Seed', a nasty ritual which causes the victim to be a contagious horror-marker, essentially. And the victims sprout lesser horrors if and when they die in battle, if memory serves. I don't recall if anybody's tested the frequency of horror-marking.

But generally speaking, unless you're sitting around in the Vale of Infinite Horror, carrying horror-marking equipment, facing astral mages who use Horror Mark or Astral Geyser, get hit by a horror sent by somebody else (horrors tend to have Astral Hooks which horror-mark you; also, non-blood forging or rituals during Astral Corruption), or cast spells which horror-mark the caster (Hell Power comes to mind)... they're not spontaneous.

Don't sit in the Vale wielding Magebane, a Robe of the Archmage, the Boots of the Planes, the Forbidden Light and the Gift of Kurgi unless you enjoy making very otherworldly and violent acquaintances.