It was about three months after Dean came back from hell. Sam and Dean continued to hunt, as the demons seemed never to cease. The angels didn’t seem to help much and Dean managed to not do all they tried him to make him do. Sam and Dean had come back from another hunt, which was both a success and a failure. What the failure meant, they came to learn in the following weeks.
The consequences started slowly, but soon the visible changes showed more rapidly. During the last hunt, they had to fight against a ghost, a zombie and what they believed was a false Santa. The Santa had tried to persuade Dean that he was the real Santa Claus, but with all their encounters around Christmas, Dean didn’t want to believe him. When the Santa seemed to become a treat and came in the line of fire, Dean shot him. After that, Sam had decapitated the zombie, not wanting to relive the last time they had to deal with one.
Dean had moved forward to check that both the zombie and Santa were dead. The Santa was still breathing, but barely, and he still insisted that he was the real one and there needed to be a replacement. Neither of the Winchesters understood what he meant. Before they could do anything else, the Santa drew his last breath. After some time went by, they figured out the truth about what the Santa had said, because of how Dean started to change.
The first change that they noticed was that Dean’s clothes seemed to shrink. Dean looked at himself more closely and all of him had increased in size. After a time, when the circumference of his thighs, arms and waist had swelled out, it continued to concentrate more around his middle and made his belly start to looking like he'd swallowed a balloon. Two weeks later, Dean had a belly that hung down over his jeans and stood out so that he could no longer see his feet. He hadn’t told Sam that he had also developed what he would say were man breasts. Actually, at first, he had been ashamed of his body's changes.
Not only did his body size increase, but Dean's appetite became larger as well, which made them need to stop more frequently. One day, Sam was woken up by a cry from Dean. Checking out what was making Dean curse and shout, made Sam work very hard to try not to laugh. Not succeeding, Dean gave him a deadly glare which followed by Dean pushing Sam away to start digging in his duffel bag.
"Don't just stand there, do you have a pair of scissors?" Dean asked when he looked up from his search.
"No, what do you need it for?"
"Can't you tell? I need to fucking cut my hair and then shave my beard." Dean walked to where Sam's duffle bag was lying.
"What's wrong with you hair? It fit you new body, Santa." Sam moved quickly into the bathroom and locked the door. He could hear Dean throw something at the door and muttering.
When Sam was finished, he listened to what Dean was doing and when it became silent in the room, he went out. Dean was lying on his bed with a bag of potato chips, a soda and a large sandwich. Sam could see that both hair and beard was still intact. Sam didn't mention Dean's little outburst, as he guessed the transformation might not yet be finished.
Less than twenty-four hours later, Sam's assumption proved to be right. At first, Dean's hair had been normal, but now it had become grey with a few white hairs streaked through it. The more that Dean had changed, the less he had wanted to get out, and now seeing the grey, he had a new fit claiming that he looked old.
"It will just fucking end with me having the red coat and trousers, and everyone will take me for Santa," Dean complained after a visit to the bathroom, trying to get his latest jeans buttoned up.
"To me it doesn't matter, I love you anyway," Sam said as he approached his brother.
Sam pushed Dean's hands away to help him, though before that, he took advantage by stroking Dean's new belly and body. He had become more attracted to Dean as he had grown.
A moment later, Dean reminded Sam that they had to find a way to put him back to normal. Dean wanted to believe that there would be something to reverse the changes without him being killed.
"But please Dean, I really like you as you are," Sam started, "and you being the new Santa, it would mean that you wouldn't need to hunt any longer. Isn't that what you wanted?"
Dean didn't say anything; he had gone to the refrigerator and took out some food which he then started to eat.
… And who knows if Dean to this day is the real Santa.