Traits

Mummy's Guise. While motionless, the Adherer is indistinguishable from a Mummy (or an ordinary wrapped corpse). A DC 15 Intelligence (Religion or Investigation) check reveals the truth.

Adhesive Wrappings. A creature that makes a melee attack against the Adherer — whether it hits or misses — must make a DC 14 Dexterity save.

• Failure: The weapon used in the attack sticks to the Adherer. A stuck weapon cannot be used; its owner can drop it for free, or spend an action to pull it free with a DC 14 Strength (Athletics) check.
• A creature making an unarmed attack that fails the save becomes Grappled (escape DC 14) and sticks to the Adherer.

Exception: attacks using a weapon that deals Fire damage, or a magical fire effect, don't trigger this feature. The sticky resin dissolves with fire.

Fire Solvent. When the Adherer takes Fire damage, all weapons and creatures stuck to it are instantly freed, and the Adherer can't use Adhesive Wrappings until the end of its next turn.

Actions

Multiattack. It makes two Sticky Slam attacks.

Sticky Slam. Melee Attack Roll: +5, reach 5 ft. Hit: 12 (2d8 + 3) Bludgeoning damage, and the target must make a DC 14 Strength save. Failure: The target has the Grappled condition (escape DC 14). The Adherer can hold two creatures Grappled at once.

How to Use

The Adherer's entire design is a misdirection. The party will think it's a Mummy. How do they handle a Mummy? Blast it from range with spells, or close in and hit it with weapons. If they do the latter — they've fallen into the trap.

Key point — the fire outlet: this creature has both Vulnerability: Fire and Fire Solvent, and Adhesive Wrappings never affects fire weapons at all. Together these three things form a clear solution path:

Use fire. A torch, Fire Bolt, a bottle of alcohol, a Flame Tongue. Fire both dissolves the stickiness and deals double damage.

This turns a "punishing monster" into a "puzzle monster." The party isn't helpless — they just need to find the right tool.

Balance Note — deviation from the original

The original problem: the old Adherer would take a martial character's weapon and remove them from the fight entirely; it never touched the caster. Asymmetric and unfair.

My fixes:

1. Dropping the weapon is free (no action required). So a Fighter doesn't end up "I'm unarmed, I lose my turn" — they drop it and switch to a backup weapon. Every martial character should have a backup weapon — this creature teaches that.
2. Grapple added to Sticky Slam: this affects casters too. A Grappled caster can still cast, but has Speed 0 and attacks against them have Advantage. Now everyone is under threat.
3. A clear and obvious fire outlet.