Disclaimer.Don't own BTVS or SPN
This is angelcest
All it took was a split second to change the course of history, a split second that happened while a vessel was being taken. Though not the one that was intended. In that second, a being was able to See what was to come and be horrified by it. As he would not lose his brother again, not after learning he was alive in that second. So he went to change it and that was how Michael and Lucifer ended up facing one another for the first time in many milenia. Lucifer having Seen it to thanks to the connection the two of them still shared.
To Lucifer it was beyond horrifying as he simply couldn't comprehend how....why he would ever even harm Gabriel, let alone kill him. His favorite brother. His littlest one. The only one he had allowed to see the true him, all of his emotions and all of his fears before the Fall. The Four of them had been inseparable and their bond had been unbreakable but Gabriel...he had made Lucifer whole. Once in Hell, Lucifer prepared to one day kill Michael but never Gabriel. The mere idea was..unbearable. It would never come to pass. Ever.... Gabriel had made him laugh no matter the situation, always ready with some sort of joke or prank on someone or something. Hew had drove all of Heaven insane with his pranks but they all loved him for it. Lucifer, Sammael, may have been his Father's Morning Star but Gabriel had been Heaven's most Favorite Son, Brother, Everything.
To Micheal it was enough. He was a good Son, a loyal and obedient Son but this was enough. Losing Gabriel once was enough, he would not do it again. No matter what he had to do, he would save his brother. For two thousand years he had been alone with nothing but his duty to his Father, his siblings and Heaven. For when Gabriel had fallen to some nameless death, he had been left alone. Raphael had..changed after Lucifer had been locked away and withdrew from the two of them. Leaving Gabriel and Michael with only each other. Even their beloved fledgling, Azrael had fallen. Azrael had been the essence of all Four of them, having a little bit of each of their Graces. He was their most precious one outside of each other. Their Father had been proud of them for Azrael.
To lose their fledgling and their Brother had destroyed them all. Michael had been to lost in his grief to see how deep it had affected Gabriel and had thus lost him. For two thousand years, he had thought that Gabriel had been to depressed to fight back and thus been killed. Now he knew that Gabriel's depression had caused him to run and never look back. Well, Michael may have lost a milenia with his Brother but he would not lose anything else with Gabriel. He would save him. No matter what. So when faced with Lucifer, he made the only choose he thought might save Gabriel.
Michael fell to his knees before his eldest younger brother and looked up at him." If I give you my life and thus everything, do you swear to leave Gabriel alone?" Lucifer looked at Michael in shock, for the first time in his entire existence he was completely stunned by what was happening before him. That Michael would....Michael was the one who always followed Father's orders no matter what, without question or doubt. Yet here he was offering up everything. It was shocking to say the least and yet while Lucifer was briefly tempted to take Michael up on the offer, not the least of which because it would mean not having to share Gabriel again, he didn't. Lucifer didn't know why as he was quite willing to kill Michael to get his victory but..not to ensure Gabriel lived.
As despite the..vision of the future, Lucifer knew that nothing would ever cause him to let Gabriel die. Especially by his hand. So he moved to Michael and rose him off his knees. The two of them talked and came up with a solution to save Gabriel.