The stat block for Mephistopheles can be found in Minsc and Boo’s Journal of Villainy. Mephistopheles is a nasty piece of work. He’s CR 27 devil who ignores fire immunity and resistance with his Hellfire mastery. Here’s a quote from the book to give you a sense of where he fits in the hierarchy of the 9 Hells:
Mephistopheles is the lord of Cania, the eighth level of the Nine Hells, and the most powerful archdevil next to Asmodeus himself. He is famed as Hell’s most powerful wizard and the wielder of a power known as Hellfire.Minsc and Boo’s Journal of Villainy, p.99
Fire is one of the most oft-resisted damage types in the game, only getting edged out by poison. For my Coliseum games, I select my monsters at random. I cringe when I pull a monster that deals poison damage. Fire is a close second. It’s nice to be able to mix things up for your players who may have gotten a bit too comfortable with their resistance to fire damage.
A Wish for Mephistopheles
Mephistopheles also has the ability to cast the wish spell at will. Meteor swarm too, but that’s not as big a deal as the first one. As a Dungeon Master, I always struggle to fairly employ the wish spell in play. The DM serves as the check on the power of wish. Casting wish as the DM has no guard rails other than the ones you impose on yourself, and the amount of shennanigans your players are willing to tolerate. I usually stick to casting a spell. I drop an extra meteor swarm, or a psychic scream. This keeps me from doing something overpowered that would be unfair.
The other option is to wish away very narrow game mechanics, with the assumption that the wisher does so only for the duration of combat. I’ve wished players were vulnerable to a creature’s main damage type. I’ve wished a barbarian wasn’t so angry, or that a wizard would lose concentration on a clutch spell. Keep the scope and duration small, and really give your players something to worry about…
Check out the other creatures who made my Hotlist, guaranteed to give your players a tough time in combat!