Slithering Lich

A high level "boss" monster: a demilich that leaps from 'body' to 'body' among a personal army of helmed horrors/golems/undead minions as a bonus action or at the top of each round. Because it can magic jar around wherever it wants PCs have to find a way to contain it, then fight it. Otherwise they can't do any hp damage to it, and it can always bring in reinforcements.

And, of course, it could also hop from actual body to actual body, turning its enemies’ allies against them.