History of Borealis Catalyst

Acquisition

The catalyst for the exotic sniper rifle Borealis drops at random from defeating non-Guardian enemies in PvE anywhere in the system.

Completion

Get 150 kills with Borealis in each of Arc, Solar and Void (450 total). The weapon's "The Fundamentals" intrinsic lets you cycle the three elements, so switch modes between 150-kill batches. Kills made before you own the catalyst reportedly still count per element.

Reward

Game text lists an "Insert the Catalyst" step first. Sources agree it boosts reload speed and adds the masterwork benefits, but disagree on the amount: +18 (Destiny Game Wiki, ProGameTalk) versus +20 plus "Shield Disorient" (Destinypedia, which may reflect a later version). Compare Hard Light, which has similar element switching.