It's called "Exalted In Passing (Fate/Fire Emblem Awakening Multi-SI Feat. Xan, Luckychaos, The Oldman, and Lord Trent Blackmore [no closing parenthesis]" (archiveofourown.org/works/43353381/chapters/108979776#workskin which is deliberately not clickable), which, aside from having an overlong title, has three massively overpowered Stus who are each a one man army, and there's even a Stu named after the person they're based on. It also looks like there's a fourth Stu coming based on the tags. On top of that, every Awakening canon introduced so far is sliding out of character, and two of the three Stus are rather clearly being set up for romance with canons.

Here is Harry Potter’s unknown sister Mina Potter. This Mina is American for some reason, is Harry’s twin sister, and has an identical lightning scar. It’s very bizarre.

—Ls would like to mission this soon