More generalised:
- Flesh render enemies have vorpal claws.
- Beholders will hit you with multiple negative levels in short order as well as using disintegrate which you'll be more likely to fail your save against thanks to the negative levels. Death ward spells will be dispelled so it won't protect you.
- High ability damage will make you helpless, which doesn't instakill you but you can't do anything except run. Except if your STR is reduced to 0 & you're carrying a lot of gear, then you can't run away either. And CON damage makes you more vulnerable to instakills as well as reducing your HP. Kopru & spectral beholders are particularly notorious for stat drain, shadow enemies can drain STR, vampire enemies will drain CON.
- Mind flayers will instakill you if they use their brain slurp after stunning you. If you're warforged, they'll throw it up again, which might give some small consolation.
- If you're warforged, every single attack from a rust monster has a chance to stun you. DC fairly low, but it can be nasty at low levels.