Multiattack

The dullahan makes three Spine Whip or Necrotic Bolt attacks. It can replace one attack with a use of Seal the Doom.

Spine Whip

Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) slashing damage plus 10 (3d6) necrotic damage. If the target is a creature, it must succeed on a DC 17 Constitution saving throw or fall prone as it is wracked with pain.

Necrotic Bolt

Ranged Spell Attack: +7 to hit, range 120 ft., one target. Hit: 21 (4d8 + 3) necrotic damage.

Seal the Doom

The dullahan points at a creature doomed by Deathly Doom within 40 feet of it that it can see. The creature must make a DC 17 Constitution saving throw. On a failure, the target immediately drops to 0 hp if it is below half its hp maximum. On a success, the target is immune to the dullahan's Seal the Doom for the next 24 hours.