Multiattack

The eater of dust makes one Maw-Arm attack and two Necrotic Blast attacks, or it makes three Necrotic Blast attacks.

Maw-Arm

Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) piercing damage plus 13 (3d8) necrotic damage, and the target must succeed on a DC 17 Constitution saving throw or its hp maximum is reduced by amount equal to the necrotic damage taken, and the eater regains hp equal to that amount. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hp maximum to 0.

Necrotic Burst

Melee or Ranged Spell Attack: +7 to hit, reach 5 ft. or range 60 ft., one target. Hit: 16 (3d8 + 3) necrotic damage.