Multiattack

The sarcophagus slime can use its Frightful Presence. It then makes two Slam attacks. It can replace one Slam attack with a use of Corrupting Gaze.

Slam

Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 9 (2d6 + 2) bludgeoning damage plus 9 (2d8) acid damage.

Corrupting Gaze

One creature the sarcophagus slime can see within 30 feet of it must make a DC 15 Constitution saving throw, taking 18 (4d8) necrotic damage on a failed save, or half as much damage on a successful one. The target's hp maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hp maximum to 0. A Humanoid slain by this effect rises 24 hours later as a sarcophagus slime, unless the Humanoid is restored to life or its body is destroyed.

Frightful Presence

Each creature of the sarcophagus slime's choice that is within 60 feet of the sarcophagus slime and aware of it must succeed on a DC 15 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the sarcophagus slime's Frightful Presence for the next 24 hours.