This skin is the Nether Lord Boss.  It is a Minecraft Boss that spawns in Nether Fortresses.  It can launch fireballs with increased explosion radius, swim extremely quickly in lava and light you on fire.  He can also give you the Wither effect.  It was made when a young Steve was trapped in the Nether.  He found a Nether Fortress and the blazes and Wither Skeletons raised him.  He learned their abilities such as launching fireballs and inflicting Wither.  Eventually his skills in combat became higher than theirs'.  He became obsessed with combat, always searching for a more difficult opponent.  He spared with the Wither Skeletons and Blazes but always defeated them until they became scared of him.  He is now the ruler of the Nether and must be defeated!