A dual-phase lightsaber is a lightsaber variation with an adjustable blade length. Darth Vader’s lightsaber is a versatile dual-phase lightsaber, according to Star Wars canon.
Dual-phase lightsabers use two kyber crystals in order to allow the wielder to lengthen or shorten the lightsaber blade length to suit the situation. The blade may be adjusted quickly, mid-combat in order to catch an opponent off-guard. The Jedi Academy Training Manual, which is part of Star Wars legends, states that dual-phase lightsaber blades could extend up to 3 meters (about 9.8 feet) in length, roughly triple the length of a standard lightsaber blade.
According to Star Wars legends, dual-phase lightsabers were most popular during the volatile era of the Jedi Civil War. Lightsabers duels between Jedi and Sith were common during the era. The dual-phase lightsaber fell out of favor as the Sith dwindled in numbers.
Darth Vader is the only confirmed character to use a dual-phase lightsaber in Star Wars canon. According to Star Wars legends, however, additional characters used dual-phase lightsabers as well including: Count Dooku / Lord Tyrannus, Gantoris, Corran Horn, Keiran Halcyon, and Desann.