The fifth-edition Dungeon Master’s Guide describes four tiers of play, supported a player character level. From level 1 to level 4, PCs are “local heroes,” saving one village at a time. At levels 5 through 10, they’re “heroes of the realm,” regionally renowned. At levels 11 through 16, they’re “masters of the realm,” on whose deeds Fates of kingdoms turn. And at levels 17 through 20, they’re “masters of the planet,” those who finish up as protagonists in books by R.A. Salvatore.
If your PCs are coming face-to-face with an empyrean, they’d better either be masters of the planet already or have excellent insurance coverage. They are, essentially, demigods. Titans. Boss monsters on par with the foremost ancient dragons. Most of them are chaotic good, residing on the plane of Arvandor, Arboria, or Olympus, counting on how old-school you wish your cosmology. But sometimes they are going on a respite bender in Tartarus or something (excuse me—Carceri), and they’re not an equivalent once they come. These depraved empyreans find yourself exiled to the fabric plane, where they take over kingdoms as a hobby. (If a 20-foot-tall god-child can’t make Posleslavny great again, who can, am I right?)
Every one of an empyrean’s abilities is extraordinary, but with beyond-extraordinary Strength and Constitution of 30, it’s a brute to rule all brutes. one hit from its maul can potentially kill a PC with up to 23 hp instantly. Its armor class, unarmored, is 22. it’s quite 300 hp. It’s immune—not resistant, immune—to physical damage from nonmagical weapons, and its own weapon attacks are inherently magical. It also has a plus in saving throws against magic. It’s not afraid to combine it up with you—even if it should be, you being a master of the planet and everyone.
Generally, then, the empyrean will favor the direct approach to handling its enemies: charge, bash, repeat. Its ranged Bolt attack, while potent, isn’t as damaging as its melee attack, and it’ll use this attack—which has the ridiculous range of 600 feet—only to strike at a distance against opponents whom it can’t yet engage in melee with. because of its Intelligence of 21, it knows, automatically, exactly which sort of injury will harm its targets the foremost.
The empyrean’s other spells are largely irrelevant; it’ll cast dispel evil and good as long as a PC has summoned one or more celestials, fiends, or undead creatures that the empyrean can’t dispatch with a Bolt or by smacking it with its maul or just ignore.
Empyreans can take up to 3 legendary actions per round, at the top of other creatures’ turns. One, Attack, lets them take a further maul or Bolt attack. this may always be an empyrean’s default choice because it makes up for its otherwise lackluster action economy (unlike most boss monsters, empyreans haven’t any Multiattack).
Bolster only applies when the empyrean has allies fighting alongside it, and if that’s the case, heaven helps your PCs, because the empyrean is plenty badass all by itself. Whether the empyrean chooses Disengage or Dodge depends on the number of enemies engaged in melee with it. If there are just one or two, it Dodges because it retreats; three or more, it Disengages.