I don't know about you guys, but I ship Marto(Marvel x Cato) quite heavily, so I do my 'research' aka I fanfiction google them.
I happened to step on this story a while ago, but it wasn't completed then. Now it is. I absolutely love it, though it didn't have the ending I had wished for. Anyway, no spoilers, you have to read it to find out yourselves :)
Eternal Damnation
Author: YoungArtist77
Formerly "Equal Grounds". "I thought I controlled the divine destiny when I volunteered because doing so gave me him, opened a door I was too scared to take. I knew destiny fought back though because by volunteering, rising above the odds, I also lost the chance to ever have him. Without me, everyone knew, he would have won." A Cato x Marvel slash based on the first book storyline.
May the read be ever in your favour :)