Linked with the Second Sword of Shadows, this Force weapon is able to hold up to any lightsaber. Supposedly, more of them exist, but this is not proven as of yet.
The sword needs to be activated in a short ritual to unfold it's true power. It holds an echo of the spirit of it's creator, a swordsmaster. When activated, the echo will create an overlay on the mind of the wielder (not taking control) and allow him or her to fight as well as the swordsmaster in ancient times – given the wielder is physically able to hold up. Used like this often enough, the wielder will take up the same skills.
The First Sword of Shadows also enables the wielder to know the moves of the opponent several seconds in advance. When used side by side with the Second Sword, the abilities of the weapons link and create a most unbeatable pair of fighters.
Placed on a specially prepared altar in a training area, the echo of the swordsmaster will invoke it's old memories in the minds of students of martial arts, enabling them to learn as if being thaught by him personally.
Any Light Sider using this sword will inevitably fall to the Dark Side.