Those are Champions. Instead of the normal champions you find on the other servers, on the hardcore server for this season they are completely new and unique to hardcore. There are 5 of them, and each one is named after a different dragon in the game. The color of the elemental sheath on them matches the elemental type they are affiliated with, and give you a very noticeable way to determine what kind of damage they'll do to you if you let them hit you, as well as what type of damage they are even more resistant to.
Kor Kaza is a blue dragon and is thus tied to lightning damage, and those champions will be wreathed in lightning.
Sinvala is a black dragon and is thus tied to acid damage, and those champions will be wreathed in green.
Velah is a red dragon and is thus tied to fire damage, and those champions will be wreathed in red flames.
Kraxxarrian is a green dragon and is thus tied to poison damage, and those champions will also be in green.
Aussircaex is a white dragon and is thus tied to cold damage, and those champions will be wreathed in blue fire.