Zalud, Guardian of Malthael is a Super Unique Winged Assassin found in Pandemonium Fortress Level 2 in Diablo III. He is one of the three (the others being Xoren and Esiel) Guardians who can spawn in the area.
Zalud always spawns at one of the floating isles connected by Death Gates, on the way to the Heart of the Fortress, so it is impossible to miss him. Killing him is not required for the quest, though.
In combat, Zalud has Electrified affix.