Frostmaster

Frostmaster is the slash ship between Grandmaster and Loki from the Thor fandom.

Canon
Thor: Ragnarok

When Hela threw Loki out of the Bifrost, he landed on the planet Sakaar, eventually gaining the Grandmaster's favor over the course of a few weeks.

When Thor was imprisoned at the Grandmaster's palace, he and Loki quickly spoke to each other, and he was confused to learn that Loki had been on Sakaar for weeks. Grandmaster told Thor that time flowed oddly on Sakaar. He stated that anywhere else, he'd be millions of years old, then gave Loki an odd look.

Loki later informed Thor of his plans to eventually overthrow the Grandmaster. Having fallen out of favor with the Grandmaster after Thor and the Hulk's escape, Loki tried to capture them again for the reward money, but Thor thwarted his plan and left him behind while he, Bruce Banner, and Valkyrie set off for Asgard.

Fanon
Fans generally believe that Loki hooked up with the Grandmaster in order to garner his favor.

Fandom
AO3