Phantom Slash is a Skill in Elden Ring. Phantom Slash is a is a regular skill that can be found in Ashes of War, it can also be used on Polearms and Twinblades (Great Spears excepted). Updated to Patch 1.07.

Elden Ring Phantom Slash Guide, Notes & Tips

• This is an Ash of War Skill, associated with the Ash of War: Phantom Slash.
• Can barely change direction when used.
• FP Cost: 8
• Deals 9+9 (Total 18) stance damage
• This Skill is Not Chargeable
• Both of this Skill's attacks can be Parried (the Phantom cannot be Parried)
• The range of both the phantom's and your own attacks will always be the same, regardless how close or far an enemy is. Due to this, the player must place themselves at the optimal range to land both hits. Better used on slower targets. 
• The phantom wears the Night's Cavalry Set and uses the Nightrider Glaive.