Cloaker

A cloaker as ally is a stretch with one good frame: an Underdark guide. The party hires a drow contact to escort them through three days of unmapped tunnel, and the contact's pet cloaker rides her shoulders like a real cloak until something happens. Cloakers speak Deep Speech and Undercommon, which means a party with the right languages can negotiate directly. The deal is shelter and a cut of whatever they kill. The cloaker's loyalty extends as far as the food.

In a fight on the party's side, lead with Phantasms before initiative if there's time, then Moan to Frighten the largest cluster of enemies. Remember the 60-foot Emanation hits everyone, including the party, so warn whoever is in range and give them the DC 13 Wisdom save. After that the cloaker dives for whichever enemy is hurting the rogue and Attaches. The Blinded target stops being a threat to the party and starts contributing damage to itself through the reflection clause. Tail attacks at +6 mop up anything that tries to detach it.

Light Sensitivity is the catch the party will forget. The cleric's Light cantrip on a shield drops the cloaker to Disadvantage on every attack, and any Bright Light shuts down a re-cast Phantasms entirely. If the party is the kind that lights torches as a default, the cloaker will pull rank and ask them to put the torches out, then sulk if they refuse. This is a creature that has lived its whole life in 120 ft. of Darkvision and considers daylight an injury.

Have the cloaker speak only when asked direct questions, and always in Deep Speech first before switching. The unsettling tone is part of the rental.

A cloaker is the trap inside the trap. The party walks into a stone vault, sees a pile of moth-eaten cloaks heaped on the wall, takes another step, and the closest one peels off and folds itself around the lightly armored member of the back rank. Run cloakers as ambush predators in dark places where bodies hang from the ceiling. Light Sensitivity gives them Disadvantage on attacks in Bright Light, so the encounter wants Darkvision-only conditions or it stops being scary.

The opening turn is Phantasms. The cloaker casts Mirror Image as a Bonus Action, surrounds itself with three illusory duplicates, then flies down on its 40 ft. fly speed to Attach. Attach hits at +6 for 13 piercing on a Large or smaller target. Once latched, the target is Blinded, the cloaker halves the damage it takes and reflects the same amount back to the host, and only a DC 14 Strength (Athletics) check from the target or an adjacent ally pulls it off. The two Tail attacks at +6 with 10 ft. reach (8 average each) hit anyone who tried to help. The cleric who reached over to pry the cloaker off is now within Tail range and bleeding.

Moan as a Bonus Action drops a DC 13 Wisdom save in a 60-foot Emanation. Failure is Frightened until the end of the cloaker's next turn, and a creature that saves is immune to that cloaker's Moan for 24 hours. Open with Moan on round one to lock down the heaviest hitters, then save subsequent Moans only for newcomers and creatures who failed before. Phantasms ends if the cloaker starts or ends a turn in Bright Light, so any Daylight or Light cantrip is a fight-changer that the party should be allowed to discover.

Cloakers are not loyal. Once Bloodied with Mirror Image gone, detach with 5 feet of movement, fly to the ceiling, and slip through the same crack the party didn't notice on the way in. The cloaker remembers them. The next vault has three.