It seems that AI cannot handle a case where it cannot reach any target sometimes. I though I'd be cute and block Horned Devil at the end of High Sewers with a Wall of Stone across the room, but this consistently leads to a crash during his turn. The same tactic worked more or less fine with whats-his-name in the room with a piano, though I think I experienced a crash there once, too. Anyway, Horned Devil's crash seem to be reproducible, so I'm attaching my save.
Steps to reproduce:
1) Go left, speak with Horned Devil, start combat
2) Wait for Driaghan's turn
3) Use Driaghan to cast Wall of Stone and block Horned Devil completely from the rest of the combat
4) Wait for Horned Devil's turn. He will attack the wall, then think some more and the game will crash