Infinity War Fix-it fic. The Avengers and their allies are losing the battle against Thanos. As New York falls, Loki finds his way into Dr. Strange's sanctuary, where Strange guards the last remaining Infinity Stone, locked away in the Eye of Agamotto.
All Loki has to do to fix it is to go back in time, all the way back to before he let the Jotun into Asgard during Thor's botched coronation, and change things. Fix the past, and he'll fix the future.
The only problem is Loki hasn't considered what exactly is waiting for him in the past. Or more precisely, who. Thor is back there, and he's even more bloodthirsty and uncontrolled than Loki remembers.