Similar to the weapon itself, you can unlock the exotic catalyst for Choir of One in the new Encore exotic mission.
If you want to obtain the catalyst, you need to compelte the exotic mission Encore first, then, visit Banshee-44 at the Tower and get the Choir of One exotic catalyst quest. This quest is called “//CHOIR: ETERNAL”
The Choir of One Exotic Catalyst comes with three different refits, Destabilizing Rounds, Onslaught, and Subsistence. Below, you can check out all three refits with perk description:
• Destabilizing Rounds Refit: Final blows cause nearby targets to become volatile.
• Onslaught Refit: Final blows with this weapon increase its rate of fire.
• Subsistence Refit: Defeating targets partially reloads the magazine from reserves.
Buy Choir of One Catalyst Boost from Boosting-Ground and leave the work to us.