Swarm of Crawling Claws

A swarm of crawling claws on the party's side is a stretch, and you should name it. The framings that work: a necromancer PC raised this swarm at level 5 and treats it as a contracted servant, a party stole the swarm's controlling phylactery from the dungeon's master and now it follows whoever holds the relic, or a friendly NPC necromancer has lent it as a one-mission asset to a paladin who will absolutely have words about it later. The horror is still the point. The party using a carpet of severed hands as a tool is a story beat about how far they have slid.

In combat, point the swarm at a single enemy and let it crawl onto them. Swarm of Grasping Hands at +4 for 20 average necrotic plus Prone is a hard opener against a humanoid bandit captain or a downed cultist. The Swarm trait lets the hands occupy the enemy's space, which is mechanically useful as cover for a friendly rogue moving in behind. Resistance to bludgeoning, piercing, and slashing keeps the swarm alive longer than its 49 HP suggests. The catch: the swarm cannot regain HP or gain temporary HP, so once it is bleeding, it stays bleeding, and once it dies, it is gone.

Out of combat, the swarm is a wedge under a door, a courier of small objects (collectively, fifty hands can carry a dwarf), and a search party. Blindsight 30 ft. and a swarm form means it pours under flagstones, into vents, and through any opening large enough for a Tiny creature. It understands Common but cannot speak, so it follows verbal commands and reports back by tapping a yes-or-no rhythm on the floor.

Have one of the hands kept by a PC after the swarm is destroyed. They will find it in their pack at the worst moment.

A swarm of crawling claws is a carpet of severed undead hands skittering across the floor of a necromancer's workshop. The horror is the visual, not the math, and you should run the encounter so the players see it before they fight it. Open with a sound: leather on stone, dozens of small fingernails ticking. The swarm comes around the corner or out from under the door at 30 ft. walk or 30 ft. climb, and the wizard's instinct to throw a fireball is right but expensive (the room is full of paper).

The opening turn is Swarm of Grasping Hands at +4 reach 5 ft. for 20 average necrotic, plus Prone on a Medium or smaller target. That is the whole offensive kit, but it is a serious hit at CR 3. The Swarm trait lets it occupy a PC's space, so it crawls onto the fighter and stays there, attacking from the same square. Knock the fighter Prone, then on the next turn the rogue steps in to help and gets dropped too. Two PCs prone in the same square with a swarm of hands climbing over them is the encounter's signature image. Bloodied (at 24 HP or less), the attack drops to 11 average, but the swarm does not slow down.

The condition immunities are the mechanical surprise. The swarm is immune to Charmed, Exhaustion, Frightened, Grappled, Incapacitated, Paralyzed, Petrified, Poisoned, Prone, Restrained, and Stunned. Hold Person does nothing. Banishment does nothing because Magic Resistance is not the issue, but most save-or-suck spells just bounce. What works is area damage that is not necrotic or poison: Fireball (8d6 in a round wipes it out), Thunderwave, Burning Hands at high level. Resistance to bludgeoning, piercing, and slashing means the fighter's greatsword does half. Tell the wizard's player early so they pivot.

The swarm is mindless and does not flee. It fights to zero and then collapses into a heap of inert hands. Have one finger twitch on the floor after the fight ends. If there is a necromancer running the dungeon, they know the swarm just died, and the next room is now warned.