The Void is a form of Light used by Guardians to fight the Darkness, and is by far the most mysterious of the current three.
In Hunters it is wielded by the Nightstalkers, firing arrows of Void Light that create anchors of Light to suppress and weaken their enemies. The Nightstalkers also use void grenades and smoke bombs to disorient, choke and disappear from their foes.
In Warlocks, Voidwalkers wield the light in a powerful blast known as a Nova Bomb. Voidwalkers drain their enemy’s life force to take as their own and use their grenades to heal themselves or destroy their opponents.
In Titans it appears as either the Defender or the Sentinel. Defenders use their light to cast a bubble shield known as a Ward of Dawn around them and their allies, empowering Guardians who step into it with Blessings of Light. Sentinels, meanwhile, cast a smaller, carried shield that they use to defend themselves and their allies and as a powerful weapon.
The Void is also theorized to be an actual place, similar to the Vex pocket dimensions or the Hive’s Ascendant realms. Whispers of an entity known as Nezarec hint at a potential being living in the Void that may be friendly toward the Light, and it is possible the Nine may have found some way to breach the Void too, using the realm hiding from the rest of the world.
Some of the most famous Guardians to wield the Void include Warlock Vanguard Ikora Rey, the Hunter Tevis and the legendary Saint-14