Zombie construct necromancer lich troll with a CR of 40.
With a little touch of shape changer, aberration, shadow, and ooze.
Acidic blood that the troll uses as a weapon (one of its methods of attack)it self harms and slings acid at the adventurers.
This should be fun, have fun with it if you want.
I think this would be an excellent end boss.
Hellfire Elemental. Take any fire elemental and add the fiendish or half-fiend template. Any fire damage it deals in half fire/half unholy damage. I made this up a while back. They are from a place where Hell overlaps with the Elemental Plane of Fire. Enjoy.