Skull Knight's Set (5e Equipment) sword
From D&D Wiki
Weapon Name: Skull Knight Sword (requires attunement) Type: Long Sword (saber) Damage: 2d8 (slashing) + 1d12 (necrotic)
Range: Melee attack
Special Properties:
Legendary Actions
Skull Knight can perform 4 legendary actions by choosing one of the options below. Only one legendary action option can be used at a time. Skull Knight regains legendary actions after a long rest.
Command Steed. The Skull Knight can make his steed
move up to its full movement. If the Skull Knight is dismounted he can use this action to make his steed move and mount it without stopping. This movement doesn't provoke Opportunity Attacks.
Sword of Actuation Release. The Skull Knight swallows his sword to combine it with all the
Behelits he has consumed. This transforms the weapon into the Sword of Actuation. The sword remains on this state for 5 minutes, after which it returns to its original
form and can't be transformed again If Skull Knight's mythic trait is active, he can use the options below as legendary actions for 1 minute after using Sword of Actuation Release.
Dimension Rift (Costs 2 Actions). Skull Knight opens a rift with his sword. This rift allows him to teleport up to 500 ft. in any direction. Aside from the knight and his mount he can bring along an additional willing creature size large or smaller.
Banishing Tear (Costs 3 Actions). Skull Knight makes 1 sword attack. On hit the target suffers an extra 9d10 necrotic damage. Whether the attack hits or not a tear to the depths of the Astral World is opened. In a 300 ft. radius any fiend or otherwise evilish creature of CR 9 or lower is automatically dragged inside. Any other fiend must make a Charisma saving throw DC 25, being dragged in on a fail. After using this action the Sword of Actuation is disabled until the Skull Knight finishes a long rest.