Mummy

A friendly mummy is a guardian of a tomb the party is trying to protect, an ancestor the party's necromancer woke up with the right rites, or a sworn protector the party freed from a worse master. Be honest with the table about why this thing isn't trying to kill them. The mummy is Lawful Evil and remembers its priesthood, its dynasty, and its grudges. It is on the party's side because their interests align right now, not because it likes them.

In a fight, the mummy is a slow anchor. Walk speed 20 ft. means it doesn't move quickly, so position it where the enemies must come to it (a doorway, a ritual circle, the foot of the sarcophagus). Multiattack lands two Rotting Fist hits at +5 for 8 bludgeoning and 10 necrotic each on a melee target, and the curse cripples enemy healing for as long as it sticks. Use the Dreadful Glare DC 11 Wis save on whoever the party is trying to lock down, because Frightened means disadvantage on attacks against the mummy and on ability checks while it can see them. The mummy will not chase. Once a target leaves its 5 ft. reach, it shuffles forward and waits.

The texture is the protocol. The mummy speaks Common plus two other languages, but it expects to be addressed in proper form, refuses orders from anyone of low station, and will not leave the bounds of its sworn duty without a binding word. Consecrated ground of a rival faith is also off-limits. When the agreed task is done, it returns to the tomb and lies down. The party can visit. The party should visit.

Have the mummy ask for one specific grave good in payment, named at the start of the alliance. Players honor up-front terms.

A mummy is a horror set piece with a slow damage engine and a curse that follows the party out of the room. The stat line looks unimpressive (AC 11, 58 HP, walk speed 20 ft.) but the mummy doesn't need speed. It needs to land one Rotting Fist, because the curse on that attack means the target cannot regain Hit Points and loses 3d6 max HP per day until somebody lifts it. That curse is the encounter.

Open in the dust, in the place where the mummy has been for centuries. Multiattack is two Rotting Fist attacks at +5 plus a Dreadful Glare. Lead with the Glare on the player most likely to do real damage on round one (paladin, the cleric about to cast Spirit Guardians). DC 11 Wisdom save, on a fail Frightened until the end of the mummy's next turn, on a success immune for 24 hours. Then close to 5 ft. and start punching. Each Rotting Fist hit is 8 bludgeoning plus 10 necrotic, and the curse stacks on every hit.

Vulnerability to Fire is the release valve. A torch, a Fire Bolt, or a Burning Hands all deal double damage and burn the mummy down faster than it can do real long-term harm. Telegraph this. Put a previous victim's body in the chamber, half-burned, with a journal note that says "fire works." The condition immunities (Charmed, Frightened, Paralyzed, Poisoned, Exhaustion) shut down most second-tier control options, so the party either burns or grinds.

The exit is the curse itself. A character reduced to 0 HP by a Rotting Fist dies and turns to dust, no death saves. Make that audible before the attack lands. The curse is the hook for the next session: 24 hours before the cleric loses another 10 max HP, and the only person who can lift it lives three days away.