Xoren, Guardian of Malthael is a Super Unique Executioner found in the Pandemonium Fortress (level 2). He has Teleporter, Knockback and Jailer affixes. Because of his affixes, it may be very hard to avoid his stunlocking attacks, so prepare accordingly. He will also most likely be located on a limited space (an island with two Death Gates). One can easily escape him, though, or even skip him altogether. There is no special reward for killing him.