A monster with a wand (or wand-like attack that requires being lined up with it) that always kills you - no matter what extrinsics or intrinsics you have, like a souped-up wand of death - but cannot or will not use it until you have seen it for a turn, to make it not an unavoidable instadeath.

Orcus is a possible candidate for this, if his wand of death is either turned into a special “Wand of Orcus” artifact or it’s just more powerful when he’s the one zapping it.