What is the name of the Norman leader who conquered southern Italy and Sicily in the 11th century?

The story behind the answer

Nicknamed “the Guiscard” (“the Cunning”), this Hauteville adventurer led the 11th‑century Norman takeover of southern Italy, toppling Lombard princes and Byzantine governors and becoming Duke of Apulia and Calabria. His campaigns opened the door to the conquest of Sicily—though the island was subdued chiefly by his younger brother, Roger I—distinguishing him from William Iron Arm (an earlier Hauteville pioneer) and from his son Bohemond of Taranto (a First Crusade leader).

The Normans began as Viking settlers in Normandy, then spread as mercenaries across the Mediterranean. This leader’s career intertwined with papal politics; in 1084 he marched on Rome to rescue Pope Gregory VII during the Investiture Controversy, a dramatic moment that showcased Norman power in the central Middle Ages.