This Gunbuddy is a part of the battle pass Ignition: Act 3 that costs 1000 valorant points. The Ruin Watcher weapon buddy is unlocked at level 44 of the premium battle pass.
Gunbuddies are charms that you can attach to any of your guns in Valorant. They usually come in pair of 2 so you can have them active in two different weapons at the same time. It’s also possible detach them and move to different guns.