Shadow

A shadow ally is unusual but not impossible. The most coherent setup is a shadow that was bound to a wizard's tower for one specific purpose (guard the inner sanctum, kill anyone who reads the forbidden book) and is now adjacent to the party because the party owns the tower or because the wizard's wards extend a courtesy to anyone wearing the right ring. The shadow does not want this. It is bound and it counts the seconds until the binding ends.

Use the shadow as a stalking horse. Stealth +6, 40 ft. walk, Amorphous through inch-wide gaps, and immunity to most of what guards a vault means a shadow can scout, distract, and kill silently in places no living scout can reach. Send it ahead in dim light or darkness and the party gets eyes on a sealed room. Send it into sunlight and it gets nothing done, since Sunlight Weakness puts every D20 test at Disadvantage.

In combat the shadow opens with Shadow Stealth as a bonus action, then Draining Swipes the enemy who matters most. The Strength drain compounds: every hit on a heavy-armor enemy makes the next hit easier and any future grapple check trivial. With AC 12 and 27 HP the shadow does not survive a focused round, so the handler should treat it as a single-use scalpel. Three turns at most.

The exit is the binding's end. Make sure the party knows what the binding is and how it ends, because the moment it ends the shadow turns on whoever is closest. The handler should be visibly nervous about a clock the party cannot see.

A shadow is a horror encounter dressed as a CR 1/2 stat block. The damage is moderate and the HP is small, but every Draining Swipe permanently reduces the target's Strength by 1d4, and a slain Humanoid rises as a new shadow within 1d4 hours. A patient pair of shadows in an unlit crypt can wipe a low-level party not by killing them in the moment but by leaving the survivors weaker and more numerous on the next encounter.

Lights are the entire fight. The shadow is Vulnerable to Radiant and has Sunlight Weakness (Disadvantage on D20 tests in sunlight), but resists Acid, Cold, Fire, Lightning, and Thunder, plus immunity to Necrotic and Poison. A cleric's Sacred Flame doubles on a failed Dex save while the wizard's Fireball is shrugged off. Telegraph this before initiative. Have the party find a half-eaten lantern oiler with a holy symbol clutched in his hand.

Open with Shadow Stealth. As a bonus action in Dim Light or Darkness, the shadow takes Hide. Stealth +6 against a party that has been operating in dim light for three rooms is plenty to land Surprise on the rogue at the back of the marching order. The shadow drifts up through the floor (Amorphous moves through a 1-inch gap), Draining Swipes the trailing PC for 1d4 Strength, and dissolves back into the wall before the party can react. Repeat twice in a corridor and the fighter is wielding a longsword at Strength 11.

Retreat is automatic. Once a shadow is below half HP and there is darkness nearby, it dissolves through the wall and follows the party into the next dim corridor. The encounter does not end when initiative ends. Keep the shadows on the sheet for the rest of the dungeon.

Roll the new-shadow timer in front of the players. They should know exactly when their dead friend is going to stand up.