History of Collective Obligation Catalyst

The Collective Obligation Catalyst is the catalyst for the Collective Obligation Void pulse rifle.

How to complete

1. Obtain the catalyst: it has a random chance to drop from Rhulk, Disciple of the Witness, the final boss of the Vow of the Disciple raid. Destinypedia says a Master difficulty clear guarantees the drop.
2. Defeat 700 targets with Collective Obligation to finish it. Use Void Subclass debuffs (Suppression, Weaken, Volatile) with its Void Leech trait to speed kills along.

Rewards

The Repulsor Brace perk: defeating a Void-debuffed target grants an overshield.

Note: sources differ slightly on drop rules (random chance versus guaranteed on Master); check in game.