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.

Nightmarish Bond. The Wraith riders can summon forth a more powerful version of the Nightmare steed, called the Specter Steed. They complete control of these creatures so long as they remain conscious. Commanding them doesn't require an action. 

Soulless Charge. As a bonus action, the wraith rider can make one weapon attack and a charge attack. If it does so, attack rolls against it have advantage until the start of its next turn.

Sunlight Drain. While in the light of day the Wraith riders are at disadvantage on all attacks and skill checks. 

Turning Defiance. The sword wraith and any other sword wraiths within 30 feet of it have advantage on saving throws against effects that turn undead.

Multiattack. You may make a two greatsword attacks.

Greatsword. Melee Weapon Attack: +11 to hit, reach 10 ft., One target. Hit: 13 (1d12 + 6) Slashing damage. 

Specter Horse Kick. Melee Weapon Attack: +9 to hit, range 5 ft., Two targets. Hit: 15 (2d10 + 3) Bludgeoning damage.

Soulless Charge. Must have at least 15 ft. of movement:  +8 to Hit, range 15 ft., One Target. Hit: 24 (4d8 + 6) Slashing Damage. 
On a successful hit the target must make a Strength Saving throw DC 20 or be knocked prone. 

The Wraith Rider can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The dragon regains spent legendary actions at the start of its turn.

Life Drain (Costs 1 Actions). So long as you have hit an opponent with your blade, you may call upon the powers of the Undead and draw life from them to the wraith. The target must make a DC 17 Constitution save or take 15 (3d8 +3). All damage that is dealt heals the wraith.

Touch of the Grave (Costs 2 Actions). As the Wraith focuses negative energy into a beam of black and grey light the target must make a DC 15 Constitution saving throw or take 1d6 worth of Strength Damage. This damage is replenished only through the use of Restoration magic. If target is reduced to 0 Strength they die. 

Create Wraith (Costs 3 Actions). If a target has died from Touch of the Grave with in the last round you may use this ability to create a Wraith. The wraith takes Two rounds to, if the Wraith rider is killed in that time the created Wraith reverts back to what he was before the ability was placed on them.