Castiel wasn't God.

Sam knew this without a doubt. Even when they watched Castiel shrug off their last attempt at stopping him without so much as a flinch. Dean had found something- an ancient weapon that should have worked. It didn't.

They drove back to the motel in silence, Dean glaring at the road the entire way back. Dean drank half a bottle of whiskey, and fell asleep.

Sam couldn't sleep. He hadn't been sleeping too well, lately. It was to be expected. He closed his eyes and saw the Cage and Lucifer, he opened them and saw the walls bleeding. Sometimes he saw faces of people he'd never met, but he knew that he'd killed them.

Tonight all he saw was Castiel- his smug, gloating expression and the corpses of all the humans he'd decided were unworthy of survival. He'd killed an entire town- for one reason, and one reason only: Sam and Dean were there to watch. Castiel didn't want the Winchesters dead, he just wanted them to watch as he killed indiscriminately. He wanted to hurt them.

Dean had given up. He hadn't said it, but Sam knew. It was radiating off of Dean in waves. They'd tried everything, and they'd failed.

Sam saw it differently. They hadn't tried everything. They'd been far too careful. Sam knew what he had to do. They had killed gods before.

Castiel wasn't God. He could be stopped. He had to be stopped. Castiel was an angel in a vessel- a vessel that had been destroyed and recreated. Castiel didn't have a soul of his own. He had his intellect, his power and his borrowed power. If souls were a consumable resource, and they were, then Jimmy was long gone. He'd been consumed.

Angels consumed human souls and used them as power, but the souls Castiel had taken in weren't human.

Sam knew a whole lot about demon souls. They'd been human once too. Demons he could pull, demons he could kill- but monsters...they were something else entirely.

Sam understood demons- intellectually, metaphysically and physiologically. His blood gave him power over them on a cellular level. Under Lucifer's care, Sam had learned a whole lot more about demons, angels and the nature of souls.

Monsters though...Sam had spent most of his life learning about them. He knew a lot about them, but it wasn't enough. Sam left the motel in search of answers, careful not to wake Dean.

The Campbells had dozens of hideouts- some more remote than others. The type Sam needed was 128 miles away. Dean would, in all likelihood, wake up before Sam returned.

As Sam hot-wired a car from the parking lot he forced himself to focus. He'd gotten very good at that over the last few months. One of the good things to come out of reabsorbing all of his memories- including his soulless year- was that he could draw on that single-mindedness when he had too. He'd need that tonight. He had a good distance to drive and he couldn't risk running off the road because he was too busy seeing everything around him burn and turn to ash while Lucifer grinned inside of him. So Sam focused, and began to recall everything he knew about Mark Campbell.

Mark Campbell was one of the few hunters who had graduated college. He had a double major in biology and chemistry obtained over six years in eight schools. Mark had more than a knack for science, he had an innate understanding of it.

Typically hunters clashed with hard science. They dealt with things that regularly defied the laws of physics, and nature itself. To Mark though, what they dealt with was science- weird and fantastic, but science nonetheless. Mark may not have been as skilled in combat as most of the Campbells, but his knowledge was invaluable. Nearly every remedy, every antidote they had was made by Mark. Sure they'd had recipes handed down from generation to generation, but Mark always found a way to improve on them.

Every hunter that joined with the Campbells had to go meet with Mark before being accepted into the fold. Mark took samples. The main purpose of this was to log all blood types in case they ever needed an emergency transfusion. The samples also served as a screening process. Mark could identify nearly any abnormality- human or inhuman.

Christian had thrown such a fit when Mark requested a sample that Samuel had to pull the two apart. Samuel told Mark to let this one slide- if Christian had anything to hide, Samuel would have figured it out by now.

When Sam joined the Campbells, his blood-work turned up several anomalies. Mark was so fascinated he brought Sam back to one of his labs, and spent three solid hours talking to Sam, trying to understand what he was seeing.

Sam told him everything- he didn't have anything to hide, after all. He'd been fed demon blood as a baby, he'd had several more gallons before inviting the Devil to possess him, and then he'd thrown himself, bodily, into the Pit. He didn't know how he came back, or why.

Mark told Sam that his DNA wasn't human. It had something extra, and it was missing something else. Sam shouldn't exist, but he did.

"Why did you drink demon blood?" Mark asked.

"For power. So I could kill Lilith and stop the Apocalypse. Turns out killing Lilith started the Apocalypse, but I didn't know that at the time."

"You ingested it. Did you ever try injecting it?" Mark asked.

Sam tilted his head in confusion, "No."

Mark turned away from the microscope and frowned ,"So the demon blood is where your power comes from?"

"No." Sam sighed, "I thought it was, but no. I tried drinking some about a month ago, just to be sure, but it didn't do anything. Whatever brought me back- brought me back different."

Mark nodded slowly, "Well you're right about that. I've never seen anything like you." Mark got up and walked over to sit next to Sam, "The demon blood didn't do anything?"

Sam frowned, "It wasn't like it used to be. There used to be this rushand then I'd be able to feel demons inside of people- I knew exactly where they were, I could grab a hold of them- pull them out of their hosts..." Sam looked up at Mark, "I could kill them."

"But now you can't."

Sam shakes his head.

Mark took a drink from his water bottle, screwed the cap back on and said, "I want to try something."

Sam followed Mark over to the large metal refrigerator in the back of the lab. Mark pulled out three different tubes, from three different shelves and carried them back over to the microscope.

"Human red blood cells don't contain DNA. Did you know that?" Mark asked, and put the three tubes down on the table next to his microscope. "But...some of our DNA is in our white blood cells."

"Okay." Sam said as he leaned over to look at the tubes Mark had selected. They were labeled "demon," "shifter" and "werewolf."

"Your red blood cells do have DNA, but it isn't human." Mark took the tube labeled "demon" and used a dropper to place some of the blood from the tube on another slide. "I want to do a few side by side comparisons."

Sam watched Mark work in silence for a few minutes, then walked back to the refrigerator and looked inside. The shelves were filled with tubes of blood and other various liquids- all of them were labeled by species. Mark had samples from everything they'd captured- and placeholders for everything they hadn't. He even had a separate compartment, containing empty tubes all labeled with a neat, little "α" followed by a species.

"You have to come look at this." Mark said.

Sam walked back to Mark and peered through the microscope.

"This is your blood. Watch." Mark said and placed another drop of blood onto the slide, "This is demon blood."

Sam watched the cells on the slide move. The red blood cells- his red blood cells closed themselves around the newly added demonic red blood cells.

"As if that isn't crazy enough, watch this." Mark said and turned one of the dials on the microscope.

Sam watched in confusion, "What am I looking at here?"

"Exactly!" said Mark, chuckling. "This is what's happening on the inside of your red blood cells."

It was odd, looking at something so small. It looked like a small electrical storm in the middle of a red sea. After a few minutes, the storm died down. Sam looked back at Mark, "What was that?"

"Your red blood cells, they uh...basically, they neutralized the demonic ones." Mark looked back through the microscope and made some adjustments, "Now look at the white blood cells."

Sam stared at the cells. They were elongated, weaving around each other, twisting together and back apart.

"They're trying to merge, and they are, sort of, but it's like...they can't complete the circuit." Mark made another adjustment to the microscope, but then there was a knock on the lab door. "We'll pick this up again soon, Sam, right?"

For a while, Sam became Mark's favorite riddle. Mark asked Sam to come see him whenever they were in between hunts, whenever they had a few hours to spare. Mark had labs set up in eleven different states, but he brought his most important equipment with him everywhere.

"You're immune." Mark said the next time Sam came to see him.

"Immune to what?"

"Nearly everything. Well, not everything, but-" Mark flipped open his laptop and pointed at the display. "Werewolf, vampire, everything viral, but some of the genetically inherited ones too."

Sam walked closer and looked at the display, "I know I'm not immune to siren venom."

Mark laughed, "True. Then again, I'm not talking about the venom. You're not immune to wraith venom either." Mark zoomed in on one of the photos, "It's the blood. Your blood, it- it overrides everything else. Whatever it is that makes a human turn into a monster- you're missing what those cells need to latch onto. The monster cells try to take over your blood, like they do everyone else's, but they can't. After a while, they just dissipate."

"So..." Sam looked at the photos as Mark scrolled from one to the next, "I can't be changed?"

"That's the theory." Mark swallowed and looked at Sam, "Want to help me test it?"

Sam stared at Mark for a few seconds and then nodded.

Sam liked hunting. He was good at hunting. Mark's tests were making him even better.

Werewolf blood heightened his senses- all of his senses- and it made him fast. Sam preferred werewolf blood to vampire blood. They had similar effects on him, but the vampire blood made him hungry, in the worst way. When he'd tried the demon blood months earlier he'd been careful. He'd remembered the pain of withdrawal and he didn't want the weakness that came with it. The weird part was, the hunger for more never came. The vampire blood though- it woke every physical hunger in Sam. Luckily, it wore off fast.

Shifter blood was interesting. Sam had hoped he'd be able to transform himself. Instead, he was able to pluck thoughts from the people around him. If he focused on one person long enough, he could hear every single thought they had.

Of course the effects were all temporary, but Mark always had something new for Sam to try. After the first few months he started giving Sam more specialized combinations. Wendigo mixed with Rugaru packed a hell of a punch. Sam decapitated three vampires that night- with his hands. Samuel saw him take the last one apart and stood stock-still, at the door of the barn, watching as Sam dropped the last vampire's head (and part of its spinal cord) on the ground. Then Samuel turned around and got Gwen's attention before she could see. Samuel tried to keep Gwen away from things- not the violence of hunting, not the monsters, just the really dangerous things- like Sam.

Sam found the abandoned strip mall easily enough, and parked the car he'd taken in the back of the building.

The entrance to the lab was hidden inside the empty greeting card shop. Sam remembered where the switch was, and flipped it, revealing a heavy metal door with pin-pad access. Sam had memorized all of Mark's passwords easily enough- it was one of the first things he did while on shifter blood, and it had been extremely useful. He had access to all of Mark's labs, and everything inside of them- even his computer files.

Sam opened the door and turned on the main light. The lab was just as pristine as it had been the last time he'd seen it. From the looks of it, nobody else had been here. Samuel had hated this place; he'd avoided it whenever possible. Sam thought it might have been the sterile environment, but it may just have been the level of technology. Mark, on the other hand, had loved it here.

Sam had been annoyed when Mark died. That had been the extent of his mourning while soulless. Now, standing in Mark's favorite lab, he felt remorse. He might have been able to save Mark, but he hadn't. The alpha shifter had snapped Mark's neck and Sam hadn't even seen him die.

If this lab didn't have what Sam needed, he'd have to drive for days to the next one in Albuquerque, and that one was a long shot.

Sam opened the first of the three refrigerators. The shelves inside weren't just full, they were packed. The first shelf was filled with case after case of monster blood. The second and third shelves had combinations tailor-made for Sam. Some he remembered, some he'd never had a chance to try. Sam took several trays out and placed them on the counter.

The second refrigerator had more of the same and also held samples of every hunter Mark had tested- except for Sam. Three of them were flagged with blue tape. Sam knew that was Mark's code for not-human. None of the names were familiar to Sam, and he wondered whether the Campbells were the reason for that.

The third refrigerator was locked by pin-pad. Sam entered the password. Inside the refrigerator were two trays full of Sam's blood samples. Mark had taken a tube or two every week so he could continue testing. The rest of the shelves were filled with samples from alphas- blood and "other."

Sam had the odd thought that Mark would have been giddy with excitement if he'd ever gotten a chance to see his collection become this extensive. After Mark's death, Sam had brought the alpha shifter blood sample here. He hadn't gotten much off of the knife Mark had stabbed it with, but it was important enough to save. He'd gotten plenty off of the alpha vampire, and he'd been able to get some from the alpha skinwalker Samuel and Christian had tracked down.

The real gold-mine, though, had been Crowley's jail. After Crowley's 'death,' Sam had fought with Dean and walked off. Dean let him cool off for a bit- for nearly an hour- which had been plenty of time to go back inside and gather blood samples from all twenty four of the alphas Crowley had captured. Eight were dead. Sam didn't know if that mattered, but he guessed it probably didn't.

Sam took a deep breath and opened the first vial of alpha blood.

The soul was the key, of course. When Mark tested Sam, Sam had been soulless. Humans became monsters body and soul. Monster blood couldn't change Sam because it needed a human soul to latch onto.

Now, Sam was whole again, he had his soul again...but his soul had spent 180 years with the Devil, his soul had been monstrous in its own right. The blood of the alphas collided with Sam's blood. He opened himself up to it and let it take root. His soul changed and grew until he could feel monsters everywhere, all around him. They heard him, they knew him, they were his- as surely as demons were. Sam called all of his souls home and they came willingly. Monster and demon alike they came to him.

Sam had killed gods before. One more wouldn't be a problem. Not anymore.

The millions trapped inside Castiel were crying out for freedom, for existence- even Purgatory was preferable to being consumed. Sam heard them all and he answered. He pulled, and the angel shattered from the force of millions of souls being torn violently free. Castiel and his vessel exploded, and this time there was nobody willing to put him back together.

Castiel wasn't God. Sam knew this without a doubt, because he knew God. In the Cage, after Castiel tore Sam's body free, Lucifer knew it was only a matter of time before he lost Sam's soul too. So he stopped tormenting Sam and started teaching him instead. Lucifer forced all of his eons of knowledge, all his memories of betrayal and fury into Sam's mind. Sam was Lucifer, and Lucifer was Sam, and Lucifer had seen God's face.

Castiel wasn't God. Sam was.