This is pretty much a trope that doesn’t really need to fit the MC or protagonist.
But, it can fit the other main characters, supporting characters or even the minor characters in the story.
Typically this trope is meant when a monster or monstrous force was once an ordinary human and finds themselves unable to return back to their human forms.
Does anyone like this trope or uses it in their story? TVTropes can explain it better than me though.
What are your thoughts and feelings on this?
I haven’t so far but it reminds me of Golem right off the bat. I think it can serve as a really fun trope for your story depending on what you’re aiming for theme wise like “they are just as monstrous as their actions/desires” type of thing.
True, true.
Hmmm… Yeah I have multiple characters who fit this category, usually as a result of others meddling. Brothers Martin is all about Tom and Steve and how their near death led to them getting experimented on by someone exploring early cybernetic integration, and through it how they both end up doing things they never would have done before the accident occurred.
To a lesser degree, Cabal - a half vampire - comes to a moment or realization that he’s always been a monster and is trying to be better than he was… (so that trope in reverse? “I was once a monster?”)
Oh this didnt go where I was expecting.
Ha! LOL! 
i love this trope! fits nicely in horror, fantasy, tragedy… very versatile.
i used it in an old story of mine about a character who became a demon. it doubled as a subtle exploration of his gender identity. demons in this story are sexless, and he didn’t conform to traditional ideas of masculinity, so part of his character arc was making peace with himself as a gender non-conforming man even when a heavily gendered society told him he did not fit the role.
ive also used it for an ai character before — whose twist was that he wasn’t actually an ai, but a human consciousness preserved on the internet. it isn’t explored quite as thoroughly with him, though.
I’ve an OC or two who were once human, no stories written about them though
One was a human of unknown gender who became a shapeshifting monster after killing an eldritch being. His “preferred form” in the present day is a masculine humanoid, though he has no memory of his human past, so he can’t say for sure if said form is supposed to resemble what he looked like before. Just says it “suits him best” when not mimicking another being or turning into nightmarish creatures (which he likes to do as a way to attack his opponents). Nowadays he’s more of a neutral entity who’d rather enjoy tea with his monster buddies over tormenting humans, as he did once. Quite terrifying if someone hurts his tea-time friends (or his sorta-adopted human nephew).
The other was a sorcerer who absorbed the power of a dream goddess and became the embodiment of nightmares due to… reasons I can’t quite remember
I pictured him as part of a group of traveling do-gooders, and on one of their journeys he was to go through things that drive him to despair and eventually decide to betray said dream goddess (who was their ally)
Of course I use this.
I have one minor, minor character that transformed into a dragon because of a curse and found out there was no way he could ever get rid of the curse.
So, he CAN transform back, but he can never live with humans. He chooses to live with dragons because at this point, he’s more dragon than human.