Harald Greycloak was the son of Eirik Bloodaxe and a grandson of Harald Fairhair. His mother was Gunnhild Gormsdatter, the sister of King Harald Bluetooth.
After his father's death in 954, Harald and his brothers allied with their grandfather, King Harald Gormson, against King Haakon I of Norway. They fought several battles against King Haakon including the Battle of Rastarkalv near the island of Frei in 955 and the Battle of Fitjar in 961.
After King Haakon's death at Fitjar, Harald and his brothers became kings of Norway, but they had little authority outside Western Norway. Harald, by being the oldest, was the most powerful of the brothers. In 961, their uncle King Harald Bluetooth of Denmark traveled to Norway and declared Harald Greycloak to be his vassal king in Norway.
Harald moved to strengthen his rule by killing the local rulers including Sigurd Haakonsson, Tryggve Olafsson and Gudrød Bjørnsson. Harald Greycloak thus took power over the country up to and including Hålogaland. Harald established control over the trade route along the Norwegian coast. He also undertook a Viking expedition to Bjarmaland, today the area of Arkhangelsk in northern Russia. Harald soon became less dependent on support from Harald Bluetooth.
In 970, he was tricked into coming to Denmark and killed in Hals in the Limfjord in a plot planned by Sigurd Haakonsson's son Haakon Sigurdsson, who had become an ally of Harold Bluetooth. Haakon Sigurdsson had become the Jarl of Lade after his own father was killed by Harald Greycloak's men in 961. The surviving brothers of Harald Greyhide fled the country after his death. With the death of Harald Greycloak, King Harald Bluetooth won back power over Norway and he supported Haakon Sigurdsson as his vassal king.
[Note: adapted from Wikipedia. Will be replaced by new text from the BP editor]