The time is set by lantern light. Dialog about coffins start the spookiness, but I understand these aren't your first 13.

The last paragraph must be the point of the question. I get frenzied confusion in the limited lantern light. Tom turns 'this way and that' trying to tell what's going on, but all he gets are fragmentary flashes. He sees the gruesome image his companion, the victim. He departs in blind panic over the bloody floor and up to the courtyard.
If those are what you intended, it worked to that extent. The extra-short sentence fragments threw me out for just a second because I was reading critically instead of reading for your intent - my bad. Once I went with them, I got the impression of fast, confusing action.

What brought him down to one knee? Why would he set down his precious lantern? These might work better after he sees Horatio, or maybe stumbles over the body.

quote:

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[“Those coffins must be down here somewhere,” Horatio said.
“If you didn’t dream them”, Tom said.<--This conversation has had no set-up for us, so I am forced to wonder: What coffins? Why is Tom accompanying him, if he doubts the validity of Horatio's claim? Do they know they are hunting vampires? All of this is important to know, for me.]
The room they were in was full of unopened crates. Tom held the lantern close to his breast. There was a dark rectangle in the wall, leading to a further cellar.
A splash of red, bright in the lantern light. A sound like sheets flapping in a stiff breeze, but the air was still. Black on black, moving fast. A wing? A cloak? [A scream that ended in a bubbling sound.<--Tom may have been a bit confused by the motion, but I'd think he'd know that the scream was coming from Horatio, if he knew where the other was.] [Moving shadows, as Tom turned this way and that.<--This basically says that Tom is moving shadows.] Down on one knee, he set the lantern on the damp floor where it shone on Horatio’s white face and torn neck.[<--Tom didn't hear Horatio's body fall? He should've seen a body, too, in the surrounding lamplight -- unless the lamp's knocked from his grasp.] [Tom heard his own feet<--From another vantage point?] smacking against wet floor and stairs. He emerged into the moonlit courtyard, gasping for air.

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Now don't take me wrong: I like this. However, this seems a little cliche' for a vamprire story -- heading down in the cellar, by moonlight, to hunt down coffins and do your vampire-exterminating? It felt a little reminiscent of Jerusalem's Lotby Stephen King.

I like the name Horatio, in spite of its Shakespearean connotations.

I like the obscurity of the motion in the darkness and the attack going unseen -- positively Lovecraftian! (Although I think Tom needs to hear more of it. Something has got to fuel the tension.)

You can mention the "wet sounds" that Tom's feet make, but the way you have it Tom isn't attached to his feet. And, if it was "wet" enough, wouldn't he slip?

I hope this has been of help.

I'll assume the reader knows something of the background here, since it isn't the 1-13. If so, first paragraph is fine to me.

“If you didn’t dream them”, Tom said.

If Tom and Horatio are the only ones in the scene, you could maybe drop Tom said. Hard for me to say without seeing the full thing, of course. Just a thought.

The room they were in was full of unopened crates. Tom held the lantern close to his breast. There was a dark rectangle in the wall, leading to a further cellar.

I think this is wordy. You could trim "they were in". That seems implied, to me. (I could be wrong.) Also, you could make it, "A dark rectangle in the wall led to a further cellar."

A splash of red, bright in the lantern light. A sound like sheets flapping in a stiff breeze, but the air was still. Black on black, moving fast. A wing? A cloak? A scream that ended in a bubbling sound. Moving shadows, as Tom turned this way and that. Down on one knee, he set the lantern on the damp floor where it shone on Horatio’s white face and torn neck. Tom heard his own feet smacking against wet floor and stairs. He emerged into the moonlit courtyard, gasping for air.

I also feel this is probably the paragraph you were mostly looking for feedback on. To me, I think you broke into it to fast, too sudden. It jolted me out of the narration, and I had to go back and read, and then see what you were doing.

My suggestion is open with a complete sentence. For illustration, something like "Tom saw a splash of read, bright in the open lantern light. Then, a sound like ..." From there, I think you could write it as normal. Ease into with one or two non-fragmented lines, then go with what you have. For me, I think something like that would work better if I were a reader.
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Those are my thoughts on it. I hope it helps in some way.

"I think this is wordy. You could trim "they were in". That seems implied, to me. (I could be wrong.) Also, you could make it, "A dark rectangle in the wall led to a further cellar."