From publisher blurb:
The Penultimate Demon!
While some books have introduced demons with a higher CR, the balor remains the most iconic demon, and its name is synonymous with immense, almost godlike power. These terrible engines of death and destruction have long served as the final challenges for high-level campaigns, and there are few words that can bring excitement—and sometimes just a little bit of fear—to a player’s eyes than knowing that the creature facing his character is none other than one of the might, fabled, and horrific balors. But even these paragons of the abyss shudder at the thought of having to confront one of their mythic brethren, creatures so strong that even mighty demon princes must think twice before battling them!
This books presents game statistics for a mythic balor, including several unique abilities never seen elsewhere. It also contains a selection of powerful balor-themed spells for use by player characters and NPCs alike, each of which also includes information for an augmented, mythic version. Finally, it includes rules for performing special rituals that can be enacted by players of any class in order to summon a mythic balor, which come with both specific game rules for doing so and detailed flavorful descriptions.