Unicorn

A unicorn arrives because the forest needs the party to do something the forest cannot do alone, and the unicorn has been sent to make the introduction. It is a Lawful Good celestial with telepathy 120 ft. and a moral compass that does not bend. Treat the unicorn like a senior NPC, not a mount and not a pet. It speaks in Celestial, Elvish, or Sylvan via telepathy, and it expects the party to listen first and bargain second.

The unicorn's combat profile is built around the bonded ally, not solo damage. Multiattack is one Hooves at +7 for 11 bludgeoning and one Radiant Horn at +7 for 9 radiant, which is fine for a CR 5 melee but not a fight-winner. The real value is Unicorn's Blessing as a Bonus Action 3/Day, casting Cure Wounds or Lesser Restoration by touch. That is three free heals or three condition removals per scene, and the unicorn should spend them on the cleric so the cleric can spend their own slots on harder problems. Spellcasting at DC 14 gives Calm Emotions to defuse a fight before it starts, Pass without Trace to move the party through hostile woods at +10 Stealth, and Word of Recall once per day for the emergency exit back to a sanctified location.

In combat, the unicorn opens with Entangle to lock down the area and Detect Evil and Good to mark the actual targets. Legendary actions are the difference between CR 5 and a real challenge: Charging Horn for a half-speed move plus a Radiant Horn at the end of an enemy turn, and Shimmering Shield for 10 temporary HP and +2 AC on a faltering ally. Spend Legendary Resistance 3/Day on the spell that would actually take the unicorn out of the fight: Polymorph, Banishment, Hold Monster.

The unicorn leaves the moment the corruption it came to address is resolved. It does not linger or accept gifts, and it makes no promise to return. If the party tries to keep it, it speaks one telepathic sentence about the forest and walks away. The grief is the point.

Have the unicorn refuse to enter any settlement of more than fifty souls. The party will start describing the road for it, and the description becomes the relationship.

A hostile unicorn is uncommon, so you need a setup. The cleanest version is corruption: a relic, a curse, a dying forest whose pain has bled into the celestial guardian and turned the Lawful Good into something righteous and wrong. Sell the corruption for one scene before swords come out. A unicorn that has decided the party is the corruption it must purge is a tragedy, and the party should know they are fighting a victim before the first attack roll.

In combat, the unicorn opens with Entangle on the cluster and Calm Emotions to suppress the bard's morale boosts. It then closes to melee with the 50 ft. walk speed (no fly, this is a horse-shaped problem) and Multiattack with Hooves at +7 and Radiant Horn at +7 for 20 average per turn. Detect Evil and Good is at-will, so the unicorn knows which PC has the cursed item, the fiend patron, or the undead under their cloak, and it goes for that target first. Dispel Evil and Good once per day is the trump card against any party member with a fiend, undead, or celestial subtype showing up on the read.

Magic Resistance plus Legendary Resistance 3/Day means the cleric's Banishment is going to fail at least twice. Save Legendary Resistance for the spell that would actually end this. Legendary actions break the fight open: Charging Horn for a free Radiant Horn at the end of the wizard's turn, and Shimmering Shield for the unicorn itself when it gets bloodied, adding 10 temporary HP and +2 AC the round before the party was about to drop it.

A corrupted unicorn does not last-stand. Once Bloodied with no obvious win, it casts Word of Recall and vanishes back to its sanctified grove, where the corruption deepens and the next encounter is harder. The party's real win condition is to break the curse, not to kill the horse. Reward them for figuring that out.

Describe the unicorn's first attack as reluctant, then escalating. Players should hesitate to swing back, and the hesitation is the encounter.