[Taqwus]

You've met one of the three forms of Umor, Eater of Gods.

Horror marks are permanent and visible, although you can only know for sure that you -are- horror-marked -- just not how badly.

"Badly"... getting smacked by Astral Geyser is worse than basic horror-mark spam. Wielding the Forbidden Light or the Gift of Kurgi is more likely to mark you than a Lightless Lantern.

There are magic sites which can spread horror marks, too... although they're not that common.