(This isn't long but I figure I'll post at least this while I try to gather the story into words. I'm a little stuck, with Sam's limited perspective.)
Alastair says nothing when Sam shows up in the morning. He hands him a bagel and the little notebook with its nubbin pencil, and wanders to the window. Stands in the beam of dusty sunlight cascading diagonally into the room, and stares out over the glass-filled alley. He half-observes Sam scribbling things down—they've stopped tracking people Sam sleeps with and have moved to recording the things he sees and the sensations he experiences under the influence of the angel blood.
Sam doesn't mind. Writing about the auras and shaded men and harsh lights and warped realities he sees helps him straighten his thoughts, and allows Alastair to explain things to him.
The motel they are staying in is seedy, dark. They've been here for almost a month, and Alastair has shown no signs of moving. Sam kind of wishes they could at least go somewhere nicer, because there's a ghost that creeps out from under the bathroom floor when Sam is high, and a lot of pulses of energy from dead rats in the walls, and the carpet has fleas. He thinks that if someone brought in a black light, they'd be horrified by whatever showed up. He certainly doesn't want to know what kind of stains there might be.
The broken mattress squeaks under his butt as he shifts in a futile attempt to sit more comfortably. He makes sure he writes down everything he can remember. When he hands the notepad back to Alastair, he says, "He's an Archangel."
Alastair nods. "I am well aware, prince." He skims Sam's unsteady notes and leans against the windowsill. (The wood is rotten, and creaks unpleasantly.) "Perhaps he can be of use."
Sam just nods. He doesn't know what Alastair means, but it's probably best to agree.
They remain silent, with only the sound of the air conditioning buzzing loudly. Such squalor has Sam on edge, and he fidgets. He rubs his face, tired, and leans forward with his knees supporting his elbows, bony. Soft sigh, and he whispers, "Why?"
"'Why' what?"
Sam gestures to himself. "Blood."
A shrug from Alastair, and the tall, spidery man slinks away from the window and sinks down into the mattress beside Sam. He leans back on his hands and stares up at the ceiling for a few minutes. Stretches his legs out long. He turns an eye on Sam to reply, "Experimentation. Prevention of possession." He lets out this long, slow smile that doesn't quite fit his face right. "Azazel wants to free the Devil to bring about the Apocalypse. I don't want that, just yet."
"What does that have to do with me?" Sam doesn't say it, he thinks it. He doesn't want to use his throat.
Alastair laughs low in his chest.
"You are the true vessel of Lucifer, Sam Winchester."
He allows Sam no time to roll his eyes. Sam knows this already—he's been drilled with it since he was ten, twelve, fourteen, so on and so forth. (Though he was told the others were also vessels... but "true" vessel seems more concrete, somehow. Like Jake was false. Ava was false. Maybe they were. Just... distractions. Competition.)
Alastair inspects his fingernails. "I want to make you as unsuitable a vessel as possible. I want to desecrate the space that he would have, and using angel blood is a way I may be able to do so. To keep him out, I will fortify your cells with angels." He stands again. Folds his hands behind his back and returns to the window. Outside, in the heat, it's begun to rain thick and dark. "I have no doubt in my mind that the release of Lucifer is inevitable, and that he will attempt to sway you. But you will not house him, and he will fail. Of that, I am also certain." He doesn't look at Sam, but Sam feels his attention firmly about his shoulders nonetheless—inside of his head. Alastair closes his eyes a moment. Murmurs, "I do not desire my domain to be flooded with bodies."
Sam chews on the inside of his cheek.
Outside, in the rain, a bird screeches.
Sam, though the most garish effects of the blood have worn away, can still see auras and the slight electric tinge to his surroundings. Alastair's face is still barely overlaid by a skeleton mask of light. That's how it is, lately. Blood, hallucinations, three days of vagueness 'til it dissipates vaguely and then blood again and the cycle repeats. Some kind of growing ability or something. It warps his view of the world in such a way that he has trouble making eye contact with, or really interacting with, anyone when he leaves the motel. Not that he often leaves. Only when necessary, or when restless.
Now, the air conditioning is laced with sparks and the window glows subtly, and the fleas in the carpet are bright white pinpricks of light. Sam's own skin glows like it's lit from within.
He twists his mouth.
Through the window, everything alive glitters with a combination of blue-white light and the wet rain. The leaves are vibrant, and spots of sun peek through the swathes of rain in thick bars, illuminating the ground. It's very pretty.
Alastair glances at him over his shoulder. Rolls his eyes. "Go ahead."
Sam smiles. Signs a quick "Thank you," and grabs his jacket before leaving the motel room to go stand in the heavy dampness of the parking lot. He turns his face to the sky—mottled with deep blue and silvery clouds—and closes his eyes. The feel of warm raindrops on his skin is pleasant. Soothing.
He sways where he stands, content.