The Mag'har orcs became playable in World of Warcraft in the Battle for Azeroth expansion, a clan from an alternate Iron Horde as seen in Warlords of Draenor. It's complicated, but basically they're like regular orcs but less green and with cooler armor.
Cosplayer Anhyra based her Mag'har armor on a design by fan artist Anatoly Zelensky (a design Blizzard liked enough to tweet about), constructing it out of varied materials. For instance, that skull on her shoulderguard is made of latex, with horns from foam clay and a base of high-density EVA foam.
You can see the full build process documented over at her Instagram, along with some more shots. Here's Anataloy's original fan art for comparison's sake.
