As a master of the arcane arts, Mephistopheles finds eager recruits among those who study magic. Any monsters that use spells, such as storm giants and oni, might follow him, and wizards' guilds and conclaves of sages are the most likely to come under his influence.
Mephistopheles's cultists can gain the Spell Shield trait. Cult leaders can also gain the Spell Leech trait.
Spell Shield
This creature gains advantage on saving throws against spells. If it succeeds on such a saving throw, it gains temporary hit points equal to the spell's level.
Spell Leech
As a bonus action, this creature chooses one ally it can see within 30 feet of it. The target loses its lowest-level spell slot, and this creature gains it.