Multiattack

The dullahan makes two attacks with its spine whip.

Spine Whip

Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 15 (2d10 + 4) slashing damage plus 10 (3d10) necrotic damage. If the target is a creature it must make a DC 15 Constitution saving throw or be wracked with pain and fall prone.

Seal the Doom

The dullahan points at a creature marked by Deathly Doom within 40 feet than it can see. The creature must succeed at a DC 15 Constitution saving throw against this magic or immediately drop to 0 hit points. A creature that successfully saves is immune to this effect for 24 hours.