Invisible Stalker

An invisible stalker on the party's side is almost always a bound servant, not a friend. A wizard PC has cast the relevant binding ritual, the party has inherited a stalker from a dead patron, or a higher-up at a temple of an air-aligned god has lent one for a single mission. Establish the contract. Stalkers are Neutral and resentful, and the players should always feel the air of a creature waiting for the binding to lapse.

The stalker's job is reconnaissance. Stealth +10, Passive Perception 18, fly 50 ft., and permanent Invisibility means there is almost no place inside a building the stalker cannot enter and report on. Send it ahead of the party into the dungeon, the manor, the encampment. It returns in an hour with the layout, the guard rotations, and the location of the thing the party is after. This is the actual reason wizards bind stalkers, and it is more valuable than any combat round.

In a fight, point and shoot. Name a target, the stalker uses Multiattack at +7, three Wind Swipes for 2d6+4 force damage, or swaps one for Vortex against a caster. The stalker will not protect the party. It will not interpose, will not heal, and will not engage a secondary threat. It does the named job, then it returns to the binder for the next instruction.

The clock matters. Most binding rituals last for days, not weeks. Have the stalker visibly remember every slight, every unnecessary errand, every time it was sent to fetch wine. When the binding lapses, the stalker should leave with the kind of look that promises this isn't over.

An invisible stalker is a contract killer the players cannot see, cannot grapple, and cannot reliably hit. The stat block does the work for you. Permanent Invisibility, 50 ft. of hover-fly, Stealth +10, Passive Perception 18, resistance to non-magical bludgeoning, piercing, and slashing, plus immunity to Grappled, Prone, Restrained, Paralyzed, and a long list of conditions that would normally let the party pin a flying killer in place. The encounter is a problem the party solves by changing the rules, not by trading damage.

Open with the kill order. Whoever bound the stalker named one target, and the stalker hunts that target until it is dead or until the binding ends. Round one, the stalker enters the room invisible, drifts to the named PC, and uses Multiattack: three Wind Swipe attacks at +7 for 2d6+4 force damage each, or two swipes plus Vortex (DC 14 Con save, 1d8+3 thunder, Grappled, no Verbal spellcasting, 2d6 thunder per turn). Vortex on a caster is the right call almost every time. It denies them their tools and pins them to the only square the stalker is in.

After the first round the party can locate the stalker by the damage trails, the displaced dust, the held character. Treat the unseen attacker rules straight: Disadvantage on attacks, advantage to the stalker, and an opposed Stealth check whenever it moves more than 10 feet. The stalker will reposition every round to break line. The fight should feel like fighting a poltergeist. The right counter is See Invisibility, Faerie Fire, Glitterdust-style effects, or flour. Reward the player who thinks of flour.

Stalkers do not negotiate and do not surrender. They serve until the contract is fulfilled or until they drop, and they resent the binding the entire time. Once the named target is dead, the stalker disengages and flies away to find the binder.

If the binder is alive, have the stalker arrive at the binder's door an hour later with a problem to discuss. The party may be invited to the conversation.