Wraith Knight

Incorporeal Movement. The wraith can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Ethereal Step. As a bonus action, the wraith knight can shift from the Material Plane to the Ethereal Plane or vice versa.

Magic Resistance. The wraith knight has advantage on saving throws against spells and other magical effects.

Multiattack. The wraith knight makes two Spectral Blade attacks.

Spectral Blade. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 18 (3d8 + 5) necrotic damage. If the target is a creature, it must succeed on a DC 17 Constitution saving throw or its hit point maximum is reduced by an amount equal to the necrotic damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Necrotic Bolt. Ranged Spell Attack: +8 to hit, range 120 ft., one target. Hit: 16 (3d10) necrotic damage.

Wraith Steed (Recharge 6). The wraith knight summons a spectral steed, a flying mount under its control for 1 minute. The steed has the statistics of a Nightmare but is immune to nonmagical damage. When the mount is destroyed or the duration expires, it disappears.