Space Allens are an alien race existing within Master Chief and The Arbiter's subconscious state before awakening in new copies of their action figures. They are ruled by the "alien king", Xanthar. They serve as antagonists in the finale of season 8's first act, "Warhead".
The Space Allens numbered in at least the hundreds, and their thoughts were primarily concerned with "raping butts", dead or alive. They boarded the cargo ship, Truth & Reconciliation, with the purpose of finding anyone on board and bringing them back to their own ships to be raped by them and their leader.
Due to previous events on the ship, the only survivor left was The Arbiter, who Xanthar had already conversed with. Despite their awareness of him, Arbiter was able to elude and kill all of the initial boarders. When Xanthar found out, he dispatched dozens more.
Eventually, their numbers became too great for The Arbiter to fight back against. They cornered him in the cargo bay, which held the ship's chemical warheads. Arbiter threatened to detonate one of the warheads with his pistol if they didn't leave the ship. One of the Allens told Arbiter that their race was undergoing a famine due to not raping anybody in days, but it is unclear whether or not this is an actual need for the species. The Allens refused to back off, and moved towards Arbiter, forcing him to go through with the threat.
The Allens cared greatly for their dead, being angry towards Arbiter for the ones that he killed. It was also implied that Xanthar has a history of punishing them for failure.