My friend wrote a fanfiction once in which Castiel has to become a demon in order to save Dean.
And this is my depiction of one of the scenes in the story.
"Dean stared at Castiel, wide-eyed, and he started to shake his head slowly. He wanted to protest, to make it stop, but a demon's powers silenced every word he had in mind.
His heart was racing, pounding painfully against his chest as horror gripped him with cold, skinny fingers, almost crushing Dean as he could only watch his lover turn into what he despised the most.
Castiel sadly looked back as his grace slipped away from him, but he was determined in his decision to save Dean, no matter what it takes.
He would do everything in order to save Dean. Always.
They locked eyes. Castiel felt his vessel's skin cracking of the tremendous force of the pure evil knocked into him, and he couldn't help but try to fight away it as long as he could.
A part of him that had already given up into the demon he'd promised to become, sneered at the obvious pain evident in Dean's expression. To the angel's own sorrow, he couldn't stop the little smirk playing on the left side of his face, taking too much pleasure in the man's suffering.
He stepped forward, letting his cold hand rest on the hunter's cheek to somehow comfort him, ease his breaking heart. He could see how this was tearing Dean apart, and Castiel had to swallow down a set of tears.
Instead, he focused on the details of Dean's face, the ones that he never grew tired of to rediscover. The pale freckles. The lines next to his eyes that increased when he smiled. The full, soft lips. He wanted to remember every single inch of this human until they both ceased to exist.
The angel grimaced in pain as another piece of grace was ripped away from him, his wings slowly disaggregating into nothing but bones, and the hunter immediately cradled his face in worry. His hands were so gentle and loving. Castiel wished they could stay like this until the end of time, just trying to soothe each others agony.
Dean's fingers curled around his trench coat and pulled him closer, not even caring that some of Castiel were distorted and down-right ugly to what it had been. Their lips came crashing together, desperate, unyielding; almost sobbing into the other's mouth, not wanting to let go.
The gruff hunter broke the kiss, resting his forehead against Castiel's, but neither could find the strength to seek eye-contact, fearing it would be the last straw before their souls would cave in, shattering into nothing.
Castiel mumbled something reassuring, but the words rang false to everyone's ears, and Dean couldn't handle it anymore, tears started to run freely, and he kissed his angel again, and again, frantic to feel those lips upon him.
His eyes were screwed shut at this point, didn't want to see the state of Castiel, knowing it was terrible. He slid his fingers through that soft, dark hair in blindness, while his other hand was clinging to the back of the coat, gripping it so tight his knuckles turned white.
The angel didn't fail to respond in the same panicked way, holding Dean in his arms, burying his nose into the man's neck to take a breath full of that familiar, comforting scent.
He was still trying to fight the inevitable with all his might although it burnt like hell fire through his body, but it was a battle he was deemed to lose. And eventually, he did.
A bloodcurdling scream wrenched out of Castiel's throat, and Dean instantly looked up to offer his support, but to his surprise all he could see was a motel room.
An empty motel room.
Needlessly, he called out for his angel, jumping up from the bed. His gaze was wild as it spun around the room, trying to find signs of Castiel somewhere, but there was nothing. It was like the man never existed.
Dean sank down onto the bed again, his heart aching, eyes stinging from tears - he was alone. His lover had given up his grace for him, given up everything, and now he was walking around as a demon. All for Dean.
It just has to be a solution to this, he thought, shivering. Then he clenched his jaw in determination; if there wasn't a solution, Dean would make one!
He got up on two feet again, moving fast to find his cellphone that laid in the pocket of his jacket. Dean called Sam, who was immensely relieved to hear his big brother's voice, and they quickly began the search for answers to get his angel out of Hell and back to normal.
Because there had to be a solution."