Multiattack. The monarch skeleton makes two dreadblade attacks.

Dreadblade. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 5) slashing damage plus 7 (2d6) necrotic damage. If the target is a creature, it must succeed on a DC 17 Constitution saving throw or its hp maximum is reduced by an amount equal to the necrotic damage taken. This reduction lasts until the creature finishes a long rest. The target dies if this effect reduces its hp maximum to 0.

Grasp of the Grave (Recharge 5-6). Each non-skeleton creature within 30 feet of the monarch must succeed on a DC 16 Dexterity saving throw or be restrained by ghostly, skeletal hands for 1 minute. A restrained target takes 10 (3d6) necrotic damage at the start of each of its turns. A creature, including the target, can take its action to break the ghostly restraints by succeeding on a DC 16 Strength check.