The one my players currently hate the most is a 2,000 year old Ghost cleric. The cool thing about ghosts is that they just don't stay dead!
They first met him in a temple and after a tough fight, killed him (they thought). Months later, he used his Malevolence ability to take over a friendly dwarf and attack a PC. Months after that, he possessed a 7 year old girl to taunt the PCs. After a while the PCs went back to the original temple, to find it reinforced with an army of undead. While the heroes were fighting the skeletons, the ghost used Malevolence to possess the Barbarian PC. It turns out that a possessed, raging, power attacking, leap attacking, critting Barbarian PC can bring the Elf Cleric PC from full health to minus 20 in a single hit.
My players really don't like the ghost cleric.
They first met him in a temple and after a tough fight, killed him (they thought). Months later, he used his Malevolence ability to take over a friendly dwarf and attack a PC. Months after that, he possessed a 7 year old girl to taunt the PCs. After a while the PCs went back to the original temple, to find it reinforced with an army of undead. While the heroes were fighting the skeletons, the ghost used Malevolence to possess the Barbarian PC. It turns out that a possessed, raging, power attacking, leap attacking, critting Barbarian PC can bring the Elf Cleric PC from full health to minus 20 in a single hit.
My players really don't like the ghost cleric.