Sakaar appears in Hulk and the Agents of S.M.A.S.H. It was once under the rule of the Leader, until the Hulks led the slaves in a rebellion and exposed the Leader.
Sakaar is an artificial planet created and ruled by the Grandmaster. A waste planet, Sakaar has no vegetation and is extremely clustered with many different aliens; its infrastructure is also made from the waste that arrives from the wormholes in the sky. Many of the inhabitants make a living however they can, be it collectors or scrappers. They do find entertainment in the Contest of Champions, a gladiatorial event held in the arena that forces captives of the Grandmaster to fight each other, of which both the Hulk and Thor Odinson were contenders for. Thor helps Hulk (in the form of Bruce Banner) escape the Grandmaster, soon Valkyrie joins their cause and supplies the gladiators with weapons to fight back, setting in a series of events that lead to an uprising against the Grandmaster's rule. After Loki Laufeyson is abandoned by Thor for attempting to betray his brother, the rebels find him and take him in as their leader, and he sets the rebels on course for Asgard.
Following the revolution, the Grandmaster is discovered by the rebels as he tries to get them to thank him for giving them the motivation to rebel in the first place.