Couatl

A couatl appearing to help the party is one of the warmest scenes in the game, and you should play it that way. Couatls are messengers and guardians on errands from the Upper Planes; they show up because something specific needs doing, they help, and they leave. Truesight 120 ft., Detect Evil and Good at will, and Detect Thoughts at will mean the couatl already knows everything the party would have spent ten minutes explaining.

Lead with the bonus action, not the bite. Divine Aid (2/Day) lets the couatl cast Bless, Lesser Restoration, or Sanctuary as a bonus action without components. Open the first round with Bless on the three party members most likely to swing this turn, then commit the action to Bite at +7 for 11 damage and a Poisoned condition until the end of its next turn. The couatl is not a damage dealer; it is a force multiplier wrapped around a +5 Wis save and Shielded Mind, which keeps it untouchable to mind reading or unwanted telepathic intrusion.

Use Shapechange off-screen and on. The couatl can take Beast or Humanoid form at will, with no concentration and no temporary HP, which means it can travel with the party as a hawk on someone's shoulder or sit through a tavern scene as a quiet stranger. In combat it usually drops the form to fight in serpent shape, but a couatl saving a captured PC might walk into a prison as a guard and walk out with the prisoner riding a snake under their cloak.

The exit matters. Once the errand is finished, the couatl thanks whoever it was sent to help, casts Sending or Dream that night with one piece of guidance, and vanishes the next morning. It does not stay for the next quest. If the party tries to retain it, it gently declines and reminds them what its actual job is.

Write the couatl's parting blessing before the session. One sentence, in character, the kind of line a player will quote back at the table six months later.

A hostile couatl is rare enough to need a setup, and the setup almost always involves the couatl being right about the party. Maybe a PC is carrying a relic the couatl was sent to recover. Maybe a warlock at the table has a patron the couatl considers an enemy of the heavens. Sell the moral discomfort for a full scene; a couatl that thinks the party is in the wrong is not going to be talked down with diplomacy.

Couatls are physically fragile (60 HP, AC 19) but they are very hard to actually hit with the things that work. Resistance to all three physical damage types means a martial party will grind for rounds, and immunity to Psychic and Radiant shuts down the cleric's smite-cantrip rotation. Open the fight with Sleep on the spellcaster (1/Day, DC 15 Wis) to remove a turn from the action economy, then Bless allies if any are present. If the couatl is alone, lead with Bite and Constrict on the same target: the bite poisons, and a Constrict success grapples and restrains the smallest party member at escape DC 13.

Use the 90 ft. fly speed to stay above melee range and Bite on dives. Truesight neutralizes invisibility tricks. Greater Restoration as a 1/Day option exists more for narrative use than the fight, but it can revive a downed ally in a single action if the couatl is fighting alongside other celestial agents.

If the couatl is reduced to a third HP and still believes the party can be persuaded, it tries Detect Thoughts on the most reasonable PC and offers a parley. If that fails, it Shapechanges into a sparrow and leaves. Couatls do not die for stubbornness.

Have the couatl name the specific moral failure it sees in the party. Out loud. Once. The fight should feel like the couatl is doing something it dislikes.