Actions

Multiattack. The Eclipse Walker makes a number of attacks. One with its Horizon Blade, one with its Echoing Cacophony and two Tortured Grasp attacks for each creature in range.
Horizon Blade. The Eclipse Walker swings its blade in a large arc, each creature within a 30ft, radius hemisphere must succeed on a DC 15 Dexterity saving throw. Taking 4d6 radiant damage on a failed save and half as much on a successful one.
Echoing Cacophony. The souls trapped within the eclipse walker continuously shriek, crying out for salvation that they cannot grasp. Each creature that the Eclipse Walker can see takes 2 psychic damage. A creature that takes this damage must succeed on a DC 11 constitution saving throw or become dazed until the end of their next turn.
Tortured Grasp. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6+2) slashing damage.

Reactions

Parry. The Eclipse Walker parries an incoming attack. For that attack its AC is increased by 3.

Legendary Actions

The Eclipse Walker has three villain actions. It can activate one each turn, during any players turn.
Eclipse Blade. The Horizon Blade's light dims, absorbing the light in the area darkening the surroundings. The Eclipse Walker teleports up to 30ft. and then swings the blade in a huge arc. Each creature within 90ft. of the Eclipse Walker must succeed on a DC 15 Dexterity saving throw. Taking 5d6 necrotic damage on a failed save and half as much on a successful one. This attack ignores cover and travels through walls.
Event Horizon. All light within 300ft. of the Eclipse Walker is snuffed out. All bright light becomes dim and all dim light becomes darkness. This effect lasts until the Eclipse Walker is slain. Each creature that starts its turn within this darkness takes 2 necrotic damage.
Psychic Scream. The screams of countless souls screech out, each creature must succeed on a DC 15 Constitution Saving throw. Becoming stunned until the end of their next turn and taking 2d8 psychic damage on a failed save and half as much damage on a successful one.