From Destinypedia, the Destiny wiki
Fallen Splicers have been busy playing with what they’ve looted from the Hive. A spike of Void energy binds to the heavy Cannon scaffold for a powerful charge-and-detonate missile.
Null Cannons are only obtainable when fighting Aksis, Archon Prime in the final encounter in the Wrath of the Machine Raid, it can be dropped by Null Captains after killing them they will drop it. However this is the only time this Weapon is available for a guardian to use.
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