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.

There should be some way for the player to get a warning that a monster has a wand of death before it zaps it at the player and kills them.

Prevent wands of death from generating before a certain level difficulty in order to prevent unfair instadeaths for early-game characters from them. This could be implemented so they only don’t generate in monster inventories and can still be found randomly on the ground, or not anywhere at all.

Charon should generate with a wand of death.

Any rock mole (including you, if you are polymorphed into one) that eats a wand of death dies instantly.

The wand of death at master skill kills and automatically zombie-enslaves the monsters it hits.