They've GOT an angel
The exorcism ritual is stolen from the tape recording Sam made in Jus in Bello, as best I and a friend who had two years of high school Latin once upon a time could manage, anyway. Probably not particularly accurate, but close enough for what I needed.
For anyone who's worried about River of Dreams—it has not been abandoned. However, both my beta and sometimes-beta have informed me that Severus turned into a complete sap somewhere in ch. 44, and immediately after I started rewriting I got stuck. And I've stayed stuck. I'm still working on it, but so far (despite a complete outline), I haven't been able to get back into the flow and make the characters sound right. Don't worry, though, it will be finished eventually.
It was going to be him that broke this seal, Castiel knew, the heavy weight of failure resting squarely upon his shoulders. Knowledge of his failure felt far worse than the pain of the lacerations and burns this body had taken. His father had set him this task, and he had fallen short. Miserably short.
Both Sameron and Lamediel had been destroyed by the demon horde before they'd even realized that the fight was upon them, and now, his powers bound by Marked manacles, Castiel was as helpless as a mortal in the face of those same demons. The mortal shell he was trapped in was chained above the altar, the doorway drawn upon the floor in front of it, and all it would take was the spilling of the shell's lifeblood to—
There was an explosion, and a demon in the guise of a young boy hit the ground with a howl of outrage.
"Dean, there's the window!"
Castiel recognized the voice almost immediately. The demon-blooded younger brother of Dean Winchester. But what either of them was doing here, almost a thousand miles from where he had last seen them….
There were snarls as the demons tried to stop Dean, but he battered his way through them towards the high window on the opposite wall with his fists and slashes from something that emitted orange light when it struck. Occasionally black smoke would pour forth from the mouth of one of the possessed, so it was a powerful weapon, whatever it was.
Sam was hidden from his sight in a mass of other demons by the entrance that they had created, but there were no telltale puffs of smoke—or the overwhelming feeling of evil—to indicate that he was using the hellsent powers he possessed. There was, however, the occasional cry of pain that indicated that he was holding his own.
Castiel had been temporarily forgotten in the melee, and he struggled to free himself. His feet could just barely reach the alter, giving him some leverage, and the wall and altar were shadowed and difficult to see from any distance. If he could just loosen the manacles even a little, he might be able to bring his powers to bear before the demons could stop him.
"Do it, Sam!" Dean shouted, leaping down from the window
Something orange and flaming shot upwards from the group surrounding Sam, and then water began spraying from miniature faucets overhead, drawing screams from the demons when the water made contact. How they had gotten holy water up there…
"Exorciamus te, omnis immundus spiritus, omnis satanica potestas, omnis incursion infernalis adversarii. Omnis legio, omnis congregario, et secta diabolica!" Sam shouted.
The demons fell back, turning towards whichever exit was closest, but the Winchesters must have done something to the doorway before they had blasted their way in, and whatever Dean had put at the window was keeping them from escaping in that direction as well. Some covered their ears, trying to block the younger Winchester's voice, but it was a futile attempt.
"Ergo, draco maledicte et omnis legio diabolica, adjuramus te. Cessa decipere humanas creaturas, eisque æternæ perditionìs venenum propinare!"
One of the demons made a wild gesture that sent Sam into the wall, and his voice faltered.
"Vade…," Dean began. "Vade, satana…vade, satana…damn it!"
"Vade, satana, inventor et magister omnis fallaciæ, hostis humanæ salutis!" Castiel called, before the demons had time to regroup. His voice wasn't as strong or as steady as the mortal's had been, but at least he could offer the boys some time. Even if it did chance drawing the demons' attention back to him.
"Castiel?" Dean left his post by the window, slashing his way through a group of demons to reach him. "What the hell are you doing here?"
Hell had a great deal to do with it, as it happened, but Castiel didn't bother to point that out.
"Humiliare sub potenti manu Dei," Sam took over again. "Contremisce et effuge, invocato a nobis sancto et terribili Nomini quem inferi tremunt. Ab insidiis diaboli, libera nos, Domine!"
"Of all the…." Dean climbed up on the altar in front of Castiel and tugged at the chains, but they were set firm in the wall.
"Does he know the full ritual?" Castiel asked.
"Who, Sam? Of course he does—he knows like a dozen exorcism rituals. He's a geek; memorizing random shit is what he does."
"Exorciamus te, omnis immundus spiritus, omnis satanica potestas, omnis incursio infernalis adversarii. Omnis legio, omnis congregatio et secta diabolica!" Sam continued, confirming Dean's words.
Dean pulled a piece of thick wire out of his pocket and caught Castiel's wrist. "Where's the lock?" He didn't bother to wait for an answer, twisting the wrist around and then dropping it and slapping his hand against the wall. "There's no lock! How is there no lock?"
"Because the bindings are fused in place."
"Ergo, draco maledicte et omnis legio diabolica, adjuramus te. Cessa decipere humanas creaturas, eisque æternæ perditionìs venenum propinare! Vade satanus, ut Ecclesiam tuam secura facias libertate servire. Te rogamus, audi nos!"
The demons' screams increased tenfold, and then there was a rush of black smoke as they exited their victims.
"Sam, get a crowbar!" Dean shouted in the silence that followed.
"What? Why? Let's just burn this…portal," Sam waved a hand at the doorway drawn upon the floor, "and get out of here before they have time to regroup! Anything that requires the ritual sacrifice of an angel—wherever they were planning to get one—is—"
"They've got an angel," Dean interrupted. "Castiel! Now get the damn crow—"
"Dean, look out!"
Sam's shout came too late as one of the exorcised victims—at least Castiel had assumed that he had been exorcised with the rest—came to his feet. His eyes were pale, nearly white, and when he snapped his arm sideways, Dean flew off the altar and slammed into the opposite wall, the back of his skull striking first. His body went limp on impact, sliding to the floor.