This Lightsaber belonged to Garen Marek also known as The Apprentice or Starkiller. It’s a single blade. It was forged by Galen Marek while under the tutelage of Darth Vader. It’s based on a similar style as his master but Marek chose to opt for a more hollow, skeletal look as opposed to the solid look. The Lightsaber was lost after Darth Vader, under orders from The Emperor, threw Marek into space in an apparent attempt to kill him.
Lightsaber by Roland Palotai of Ro-Lightsabers