Planetar

A planetar showing up to fight on the party's side is a strong scene if you handle two things. First, it's on loan from a deity or higher celestial, and it returns the moment its work is finished. Second, it does not improvise. If the players try to extend the visit or ask for help with a problem the planetar wasn't sent for, it politely declines and gives them a one-line blessing.

Use the planetar for one specific thing. A paladin might invoke a god's intervention. Sometimes the party simply walks into a location where the Upper Planes have stake in the outcome. Whatever the reason, the planetar should arrive with a stated purpose, which defines the scene's beginning and end.

In combat, lead with Holy Burst once for the dramatic clear, then identify the most evil creature on the field and go for that one until it drops. Detect Evil and Good is at-will, so the planetar knows without asking. This is your chance to mark a villain as definitively evil to the players, since the planetar will not bother with shades of grey. If your actual BBEG is morally complicated, the planetar might hesitate, and the hesitation can carry a whole subplot.

The exit matters more than the fight. Once the threat is gone, the planetar says something brief and uplifting to whoever summoned it and leaves the way it came. Don't let it linger. The party should feel the loss of having had something perfect on their side for one scene.

A trick worth doing: write the planetar's farewell line before the session. A clean exit beats five minutes of awkward improvised goodbye.

A hostile planetar is uncommon, so you need a setup. Perhaps it has been deceived about the party's intentions, or it serves a god at odds with the party's patron, or a corrupted relic has tipped it off-balance. Sell whichever setup you pick for at least one scene before swords come out, because a planetar that's wrong about the party is a tragedy, and a planetar that's right about them is a campaign-ender.

Once the fight starts, the planetar is an executioner. Its walking speed is 40 ft. but it hovers at 120 ft. of fly, so it opens with Holy Burst from outside the party's effective response range. The DC 20 Dex save and 24 radiant in a 20-foot sphere will drop several casters and squishies in one round even on a passed save. After the burst it drops into melee for three Radiant Sword attacks, averaging 96 damage on full hits. That's enough to put a tank on the floor if the dice cooperate.

Spellcasting is your difficulty lever. The planetar always knows who to hit thanks to Detect Evil and Good. Save Dispel Evil and Good for a player who's clearly an undead or fiend, such as a warlock with a fiend patron, for a dramatic moment. Commune is your release valve. If the fight is going badly, the planetar withdraws via flight and reports back to its superior. Planetars do not do last stands.

Magic Resistance plus a 21 Passive Perception keeps surprise rounds and disabling spells largely off the table. The fight should feel hard but legible. The party should always understand why the planetar is doing what it does, even as they can't stop it.

Telegraph the Holy Burst with a line of celestial song before you call for the save. The pause between the song and the save is half the encounter.