[Kobal2]

Both are true.

Any character which has been Horror Marked by any mean has a chance to get randomly attacked by a random Horror out of the blue at the start of every turn. The more marked they are, the higher the chance and the beefier the Horror. There's no stopping them from coming, or cleansing the mark. That guy is a dead man walking.

Most Horror attacks also mark their targets, so even if they survive, they're eventually boned - but this is not the only way to catch a Mark, which is perhaps what is confusing you. You can also get marked by some regular battle spells, and by carrying certain items (such as Lightless Lantern)

You can *also* summon Horrors in battle or through rituals at which point they'll proceed to destroy both sides, but this is a separate mechanic. Those summoned Horrors will still home in on any marked character present first, so you can do a round of marking before summoning them to get better odds that the summoning won't go too pear shaped for you. Or you could just send a small, unsupported mage squad to spawn a bunch of Horrors on an invading army before retreating to watch the fireworks from a safe distance.