Loki is good (I think) and theres this beast/alien cursed like one from Norse Mythology where the person who kills it (Thor in the myths) takes 9 steps and dies. Basically Loki stops Thor from killing it and does it himself and actually dies (but comes back I think). Im pretty sure the Frost Giants were there as well, battling some army with the rest of the Avengers? They may have been shapeshifters and for some reason I think the aliens were called Dire Wraiths.Its possible it was also a crossover but I dont remember what with.
With all that you'd think it'd be easy to find but for the life of me I cant. I hope someone recognizes this fic. Im going to feel like such an idiot if someone finds it, I just know it.