Intellect devourer. Most level 20 PCs have a +0-2 int save.
If the devourer wins initiative, 25% chance the PC fails the save then has its brain eaten before the PC can act.
A 10% chance to die when fighting a low single digit CR monster. Like 35% if the devourer surprises the PC.
Fighters, Monks, Wizards, Paladins, Druids, and Arificers are going to be nigh-immune from saving throw bonuses. Some builds, like Rangers, Barbarians and thise that take certain feats/items, are going to be surprise immune/win initiative. Casters who keep up foresight, or have a simulacrum backup capable of casting wish (or, in fact, any non-trivial minion stronger than a familiar: just kill it before it eats the PCs brain), are also going to win.