The PvP Legendary Backpiece?
You get it by ranking up league, doing pvp achivement and a few mats(like less than 10 gold). So basically you get it by playing alot.
You play matches to go up the PvP league and get tickets. You buy what you need from the vendor. And you get another gift by completing the 4 meta achievements and combining them in the forge.
Check the metaachievement under the Competitive tab and you’ll know what you need to do.
It is a back item to use where ever. Dunno what made you think otherwise. Its exclusive to PvP since you can only earn it in PvP.
I got my baby KFC Golden Chicken Wing back piece last night, well, the very start of the process simply by grinding out ranked games until I reached Div II. That alone rewards you with enough tickets to purchase the first tier of it.
I guess the rest of the process is either time gated or locked behind a plethora of PvP achievements.