The lightknife was a custom short lightsaber akin to a vibrodagger in uses. Much shorter than a normal blade, it was even smaller than a conventional 'Shoto' short lightsaber. The blade was 15-20cm long, and it also had a shorter hilt, which could be concealed more easily. The hilt was adapted to be in length equal to the width of the user's hand, requiring the use of tighter compression of components and a smaller power cell. The power cell would also need recharging more often than that of a lightsaber.
The only known lightknife was produced by Jobie Wan Kenobi, manufactured from the components of his old Sith lightsaber. He used it as an off-hand weapon in conjunction with a Nightscythe or normal lightsaber.