Stag-Heart Skull Sallet is a legendary hat that can only be bought from the in-game store
An NVIDIA Account is required to redeem this reward
Giveaway here: https://www.nvidia.com/en-us/account/redeem/main/
Then redeem the code on your region ESO web page
Redeem HERE for US: https://account.elderscrollsonline.com/en-us/redeem-code