To unlock Ruin in Black Ops 4 Blackout, you need to find the “Punching Mitts”, defeat 1 enemy in close quarter combat and finish the match with the Punching Mitts in your inventory. If you fail to complete the character mission for any reason in one match, you need to find the Juneau’s Tags again in the next game again.
The Punching Mitts mission item can spawn in 3 different locations on the map. It will change randomly each match.
ALL PUNCHING MITTS LOCATIONS
On top of the north crane in Construction Site
On top of the Lighthouse
On the roof of Asylum
After finding the Mitts, second requirement for Ruin is taking out an enemy in close quarter combat. This can be done in any fashion you want with any weapon, not only melee. Important is only that you are relatively close to your enemy.