This was the personal dual-phase lightsaber of Darth Vader, a Sith Lord who served as the third and final apprentice of Darth Sidious and supreme military commander of the Galactic Empire. He built the weapon some time after his fall to the Dark side of the Force at the end of the Clone Wars. It replaced the one he lost during his duel with Obi-Wan Kenobi on Mustafar.
Because of the modifications Vader made to his lightsaber over the years, its exterior also changed, somewhat, in appearance. After his accession to Sith Lord, Vader was known to draw his lightsaber when facing Jedi, but also against other opponents, even though he sometimes preferred to use the Force to dispatch lesser victims.