The fifth-edition Dungeon Master’s Guide describes four tiers of play, based on 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 the fates of kingdoms turn. And at levels 17 through 20, they’re “masters of the world,” the ones who wind 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 world already or have very good health insurance coverage. They are, essentially, demigods. Titans. Boss monsters on par with the most ancient dragons. Most of them are chaotic good, residing on the plane of Arvandor, Arboria or Olympus, depending on how old-school you like your cosmology. But sometimes they go on a spring break bender in Tartarus or something (excuse me—Carceri), and they’re not the same when they come back. These depraved empyreans end up exiled to the material 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. A single hit from its maul can potentially kill a PC with up to 23 hp instantly. Its armor class, unarmored, is 22. It has more than 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 advantage on saving throws against magic. It’s not afraid to mix it up with you—even if it should be, you being a master of the world and all.
Generally, then, the empyrean will favor the direct approach to dealing with its enemies: charge, bash, repeat. Its ranged Bolt attack, while potent, isn’t as damaging as its melee attack, and it will 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. Thanks to its Intelligence of 21, it knows, automatically, exactly which type of damage will harm its targets the most.
Hypothetically, an empyrean might use Bolt to shut down a spellcaster who could cast spells that bypassed its immunities and resistances. The thing is, thanks to its spectacular saving throw modifiers, there aren’t any—with the possible exception of earthquake or meteor swarm. Even against these, despite having a Dexterity saving throw modifier of only +5, it still has Legendary Resistance, allowing it to leap clear of the fissure or take only half damage from the meteors.
Like other monsters with Legendary Resistance, which grants three automatic successes per day on failed saving throws, the empyrean will rarely, if ever, fail a saving throw; if it does, it will probably be a Dex save. Consequently, it won’t bank its uses of this feature. It will use it any and every time it fails a save, until its uses of the feature are exhausted.
Once per day, an empyrean can cast fire storm, which does 7d10 fire damage on a failed save, half that on a success, and ought to be able to engulf every one of the empyrean’s opponents. It won’t lead with this spell, nor will it wait until the last minute to cast it. Rather, it will cast it when it realizes it has to take its opponents seriously, i.e., when they’ve managed to moderately wound it (reduce it to 219 hp or fewer).
Earthquake, on the other hand, it will cast as soon as combat begins, because it can sustain this spell, probably for the entire duration of combat, and it offers the empyrean nothing but advantages: creating difficult terrain (which the empyrean can effortlessly fly over—in fact, it will probably spend the entire combat encounter floating a foot or two off the ground), distracting spellcasters who are trying to concentrate, knocking enemies prone, opening cracks in the ground that enemies may fall into.
Plane shift, as usual, is an escape hatch, which the empyrean uses when it’s seriously wounded (reduced to 125 hp or fewer). Players, of course, will find this profoundly unsatisfying; at the highest adventuring levels, however, PCs will probably have access to plane shift themselves and thus may be able to pursue, if they can determine where the empyrean has gone.
The empyrean’s other spells are largely irrelevant; it will cast dispel evil and good only if 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 simply ignore.
Empyreans can take up to three legendary actions per round, at the end of other creatures’ turns. One, Attack, lets them take an additional maul or Bolt attack. This will always be an empyrean’s default choice, because it makes up for its otherwise lackluster action economy (unlike most boss monsters, empyreans have no Multiattack).
Bolster only applies when the empyrean has allies fighting alongside it, and if that’s the case, heaven help your PCs, because the empyrean is plenty badass all by itself.
Trembling Strike is a crowd-control feature—actually something of a desperation measure, since it costs two legendary actions, and all it does is knock opponents prone. By inference, its purpose is to manage the number of enemies the empyrean has to fight at one time (especially relevant if you’re using the optional Flanking rule—Dungeon Master’s Guide, page 251). It fears nothing, but fighting half a dozen enemies simultaneously, as Fezzik would tell you, comes with its own complications. Trembling Strike has a radius of 60 feet, so it’s overkill against most adventuring parties, and the empyrean will probably bother only if all its opponents are in the area of effect. Against a party that’s brought a bunch of its own minions along, though, its standard differs: it will want to catch either all the PCs or at least a dozen enemies—it doesn’t matter which dozen—in the area of effect.
As mentioned above, the empyrean would rather scram than die. Also as mentioned, PCs powerful enough to seriously wound an empyrean are powerful enough that they may be able to pursue it when it plane shifts. An empyrean that’s seriously wounded but has already blown its single daily use of plane shift could Dodge, Dash or Disengage, and any of these might be feasible; in practice, though, I think it would favor Dodge or Disengage over Dash. This is because the empyrean already has a movement speed higher than almost any PC can match without magical enhancement (a barbarian with the Mobile feet could just barely keep up). What are the chances, if the PCs have magic that lets them travel at more than 50 feet per round, that the same magic won’t also let them travel at more than 100 feet per round? The empyrean would Dash away only if it knew for a fact that its enemies could run it down if it moved away at its normal speed but not if it moved away at twice that. (With its Intelligence 21, however, if this is in fact the case, the empyrean knows it.)
Whether the empyrean chooses Disengage or Dodge depends on the number of enemies engaged in melee with it. If there are only one or two, it Dodges as it retreats; three or more, it Disengages. Either way, retreating isn’t the same as quitting. It uses Dodge or Disengage as its turn action, but with its legendary actions, it continues to lob attacks back at its pursuers (this is the other thing that its Bolt attack is good for), and if it’s being chased by a dozen or more enemies at once, it uses Trembling Strike to stop them cold.