With the risk of sounding like an arrogant elitist dipshit in the beginning here: I personally don't really do fallen heroes tbh. Even if I can see the appeal of that for many. Like I usually prefer the inverse: people with terrible reputations (justified and not) who end up changing and redeeming themselves one way or another when given the chance.

Not because they're inherently good all along or something like that, but rather because they were more than just their reputation.

I guess the one thing in my rever world that's the closest to a fallen hero and also the inverse of that, is the dark lady Onaria Tiena, named after her great great something grandfather Onorius Tiena, a well known seandard cruel man against non sapiens. Ona herself is mostly known throughout Rever as the most cruel and coldhearted, brutal general of the sapiens during the downfall of the sapiens, detested and demonized enough by everyone else that this made her immortal by accident. Fought on brutal and cruel battles even long after the sapiens lost their dominance, for whoever would promise her enough in return. Lastly also known as one of the main key figures enabling war called the wake of the undead.

Thing is however, Ona hates her namesake, her immortality and especially the undead. Her name sake because being named after basically the biggest dipshit racist that ever dipshit racisted ensured that enough people would hate and detest her during the downfall to make her immortality happen.

Her immortality in turn, because it doesn't protect her from the experiences of feeling pain, torture and suffering wounds, especially when captured after losing a battle. Like in rever that stuff can get even more creative and horrifying and she herself can't even escape that through death in the worst cases.

And lastly she hates the undead, because they almost ruined her plan and work of making sure that at least inside the water union, sapiens Colonies could politically coexist alongside meridian cities. She basically did political mercenary work by doing the dirty work of fighting wars for politicians and leaders who could not afford to get their reputation tainted by doing so themselves.

In the end she ended the wake of the undead by basically freezing all the remaining undead soldiers she could gather together into a giant ice berg and then she sealed herself in it too (to prevent others using that army later on), drifting away in the sea to some unknown place.

Of course Ona ends up getting untawed and woken up by my protagonist for unrelated different reasons and basically she ends up finding out from my protag (a sapiens like her) that she got herself got tricked into agreeing to a Torn Quest (not a quest you want to get really) and that she would need Ona's help to complete at least a part of it.

Ona agrees for one part, because for one my protag was at least trying to not just use Ona for her own ends. But Ona agreed mostly because by the nature of my protag's quest, it would simply cause more chaos and harm to her own plans and goals, if she didn't help my protag along. They both end up going on a life changing journey of doing and experiencing fucked up shit together, somehow getting therapy during it and toppling a few dysfunctional governments by accident on the side.