[markmayhugh]

I definitely should have looked in the manual. Robe of Magi is the culprit for my Pretender, and the Ankh is the culprit for my Hero. I will make a thorough list and email it to Edi to update his database.

The Demon Gate could also be a factor. I wonder if it is supposed to horror mark, even though the database does not include it?

Slight chance may be understated. I put both of those artifacts on their respective characters just two turns earlier, and they both got horror marked on exactly the same turn 2 turns later.