Probably some mega bug. There's no hope of getting it fixed in IVANX, but it'd be worth checking the community version to test if that bug can be reproduced there
I was doing a goofy little run on IVANX when I stumbled upon an altar dedicated to Mortifer, since I was polymorphed into a ghost, I decided to pray to him since he couldn't cause too much damage to me that way. Well, the first time was a no biggie, he just broke a shield I wasn't using and poured water on me. The next time however, was interesting. As soon as I hit 'y' to pray, the game locked up, and then crashed, with my save being deleted alongside it. Is this just a bug of IVANX? Or is it an actual feature in this variant?