Hidden Artifacts: Unlocking the 4th (PvP) color scheme
The 4th tint of hidden artifact appearances requires 1000 PvP kills to unlock.
Now, I am not sure if this is common knowledge already, but those 1000 player kills do not need to be honorable kills. Killing a level 85 or even a level 18 contributes the same way to unlocking the PvP tint as killing a level 110 does. (could not test it below that due to faction-controlled zones)
I'm pretty sure they also count from arenas (someone feel free to prove me wrong I'm not 100%) because when I got the skin all I had done was skirmishes/arenas. Wanted to farm the hks, did maybe 2 bgs max? and got the skin passively