It drops from vox obscura Chain of command is a good alternative, it has a decent fixed roll

Why would you want it when Archon’s Thunder is a much better stasis LMG?

As the last person said it can drop from completing Vox obscura, I haven’t seen it appear before but it should have a chance to appear in the weekly gunsmith offerings or Xür’s inventory. Considering how many seasonal weapons there are I wouldn’t hold your breath for it appear in their inventories anytime soon. But for future reference here is where you can obtain certain seasonal weapons: All: a chance to appear in the weekly gunsmith offerings or Xür’s inventory (exclusives: Witch, Deep, plunder, chosen, Lost, and Hunt weapons) Vox Obscura: Risen weapons Serph shield: serph and worthy weapons Presage: Haunted weapons Override Avalon: Defience weapons Starcrossed: Wish and undying weapons When edge of fate comes out: Encore: Echos and Dawn weapons Kell’s Fall: Revenant and splicer weapons Derealize: Heresy and Arrivals weapons