Malthael is the big baddie from the sole Diablo III add-on called The Reaper of Souls.

He once served the Heavens, but didn’t like the style of conflict resolution so he decided to end the never-ending wars by himself. Basically, by becoming very, very bad. He’s the hero’s spotlight for you to enjoy:

The Reaper of Souls is a melee hero and he’s role will be eliminating opposing tanks. Malthael is available right now, if you’re itching to try him out and until June 26 players can earn rewards both in Heroes of the Storm and Diablo III in connection to the introduction of the new character.