Could do Beast!Reader and SilentFarie(Silent salt x Elder farie)? Reader being their ex-lover and becoming corrupted, maybe becoming the beast of perfection (or something like that :D)
TRIGGER WARNING FOR MISTREATMENT, CORRUPTION, DEATH, AND JUST GENERAL SAD AND DEPRESSING THOUGHTS AND SPOILERS FOR BEAST YEAST!
You were a beast, one of the great heros of the past. Back then you were the Beast of Goodness.
Now, this doesn't seem uncommon amoung the Beasts but yours was unyielding and infinite. The cookies who met you said they felt a warmth in their soul even long after you had left. This kindness wasn't just limited to the common cookies of Earthbread, this kindness extended to your fellow Beasts and they all loved you for it, especially the Salt of Solidarity.
However, the Salt of Solidarity was conflicted. At this point, he was already in a relationship with Elder Faerie, and he was happy. But, that burning love he felt for Elder Faerie he started to feel for you. He felt ashamed, thinking that he was becoming unfaithful but on the other hand he still loved Elder Faerie.
This didn't go unnoticed by Elder Faerie Cookie, he could tell that something was wrong with his love. So, one night while the two sat in a flower field, Elder Faerie confronted the Salt of Solidarity.
"My love, what weighs on your heart?" He asks.
Finally unable to keep his feelings hidden, the Salt of Solidarity told Elder Faerie all about his feelings for you and how ashamed and confused he felt. Elder Faerie listened, waiting until the Salt of Solidarity had finished before pulling him into a hug. Elder Faerie comforted his love, thanking him for his honesty.
"You are not mad?" The Salt of Solidarity asks.
"No, I am not," Elder Faerie says.
***
A week after that night the Salt of Solidarity decided to introduce you to Elder Faerie. The moment Elder Faerie met you he felt something deep in his dough and jam, the burning, infinite love he and the Salt of Solidarity held for each other. Seeing your kindness made him understand how his love could fall for you. So, they invite you over more often and while you think you're intruding they reassure you that you're not. These visits go on for years.
Then, one night the pair invites you over as normal. This time however, there was a table with glowing candles, delicious foods, and your friends. You sat, asking what the speical occasion was with a chuckle and smile.
"We have somwthing important to ask you," Elder Faerie says.
"Oh, what is it?" You ask.
"We have been discussing our relationship and we have both realized we love you and each other. So, we would like to ask if you love us the same," the Salt of Solidarity said.
Your heart raced. You were thankful they both loved you, as you had been gainning feelings for them but you didn't want to ruin their relationship, so you said yes.
After that, you three were a wonderfully happy trio. You couldn't remember the last time you'd been this happy in your life, and you wished it could last forever.
Unfortunately, fate doesn't like forever. Your fellow Beasts started to show sighs of a darkness within their souls. You tried to get through to them but it didn't seem to work, and then you noticed the change in your love.
You tried to confront him but he was too far gone, just like the others. You held out for as long as you could, but the cookies didn't make it easy. No matter how much you gave it was never good enough. You had found yourself three powerful cookies to be both companions and helpers and that kept you sane... Until that night.
The youngest cookie you had found, one who reminded you of Eternal Sugar Cookie, was murdered. You remember that night clearly, seeing her jam flow across the ground while the cookie responsible just stood there and no one tried to help her!
Elder Faerie remembers the air that night, it became cold yet electric all at once. He wasn't able to see what had happened but he saw the outcome, you standing there with an empty smile and a landscape that moved in unnatural order, it was too perfect... That's what you became, the Beast of Perfection.
When Silent Salt saw you and your new drive he didn't try to stop you and with that, all of Earthbread's heroes had given into darkness. Elder Faerie Cookie was devastated when he finally saw you in your new form, the kind cookie he spend centuries loving was replaced by a cold tyrant.
Your trio had fallen apart when Silent Salt changed, but Elder Faerie held on that maybe it could be you two and that you could bring him back! But, that didn't happen. He had tried to reach out to you, his last attempt to bring back the cookie he loves.
"Please! My love you cannot do this!" He begs, tears falling down his face.
"My my... we've been causing quite a scene, haven't we?" You coldly responded with a smile.
He shouted back that he wasn't causing a scene, he was trying to help you so you could help Silent Salt! But you just laughed and looked at him.
"Aw, yes, Silent Salt- his impurities soak up all of the warmth in his Soul Jam. He thinks he needs us," you said.
"What are you talking about? He just needs help! If you think that's a flaw than I'm flawed!" He yelled.
"Poor Elder Faerie...his impurities absorb all the blue in is light. He's so strong, but so weak when it comes to me and Silent Salt. As for me, I'm certain I don't need you. After all I'm every color of the light! But you're a part of me...the part I always have to repress," you say, acting as if he's not even there.
Elder Faerie Cookie didn't know whether to be offended or shattered at the way you spoke, acting as if he's and Silent Salt where just flaws in your world. He was so upset he wanted to fight you, but he couldn't. In shock and despair he just stood there as you left him. Now, he had no lover, he was alone.
***
When you and the others were sealed away, he sobbed in private. His loves were so close, yet they weren't the cookies he knew before. He'd visit every day, hoping that maybe he could help you both become close to what you both once were.
Then you were all freed. He saw how you didn't even stop to look at him, not even a glance of disgust, just pure, cold indifference. He wanted to cry out your name, reach for you, but he couldn't. He had to protect his kingdom, but he didn't last long.
As he died at the hands of Shadow Milk, he couldn't help but let out tears of despair, not at dying, but at how you looked at him.
That look was cold, empty of all emotions. You didn't even say anything, but he could feel your judgement and condescension. He could almost here you telling yourself that you were right, he was flawed and looked what happened while you're perfect and you're free!
As the icy grip and darkness of death grabbed him, he reached out for you and Silent Salt, one last time and he watched as you both ignored him even as he whimpered and lightly cried.