There are no large heaps of material save for in the far corner.  The stones and exposed earth underfoot are dark in color, making it impossible to tell from a distance where blood or other material may have been spilt.

He asks the tobacconist, “Did you observe any bits of blood or black matter earlier in the lane or outside this place?”

“Hm?  Can’t say I did.  We could certainly smell it, though.  Even as far out as the pump.”

He steps slowly towards the wall with his eyes cast downwards. He does spy some spilled blood further out, at least, even if there is none of the ridgy stuff.

Is the darker material the innards of his enemy? Or could these leading blood spatters have come from either himself or the enemy? There was hostile contact, at least, away from that corner. Since he was ultimately the victim, presumably it was he who was wounded and was fleeing pursuit until reaching this unfortunate dead end.

He approaches the gore gathered about the wall. Using the spike end of his axe, he lightly taps into one of the larger bits.

What is it? How does it respond to the spike?

The material is spring-like and elastic.  Where he has pierced the surface, he reveals blood still wet underneath.

If he pries up a section of the outer layer he’ll see that scattered within are fragments of bone and what he thinks is brain matter.

So the material is a mix of his own body’s flesh with something foreign: either a weapon of the enemy or the ichor or vital stuff of it. 

Well. Not for certain. It could also be some final self-immolating strike of his own powered by the Art… though if he had known last night a technique that would create such results (it’s definitely not Fireball, Snilloc’s Snowball Swarm, nor Blight or any other lesser necromancy), he has since forgotten it.  

“Describe what happened after you wiped down my body.”

“Well, sir.  I had to drag it out of the corner by the arms to have any shot of getting it clean.  Thought there might still be danger about us, so I had my man draw arms and keep a lantern held high, watching the way behind us. I suppose he sent off the boy with a few commons and a scolding.  

“Once you were clean enough, with more dirt on you from the turning than blood, we loaded you up.  Had to unload the other deaders there,” he points back to the corner of the lane they turned in upon, “on account of you being too big.  Wouldn’t have stayed atop that pile a yard with how slicked down you were.”

“When you were moving my body did you feel anything that was either too hot or too cold to be a recently killed man?”

“Water was cold, and my hands were stiff by the time I’d wiped you down.  I don’t rightly remember noticing anything else about it.”

“I’m glad to hear that.”

“And this ‘foundry’ odor about us, as you describe it. Did the scent cling more strongly to this place when you first arrived than it does now?”

“Oh, to be sure.”

He nods.  “Well then.”

He points down at the mass in the corner. “Look here. The flesh parts of the enemy, mixed there with my own, even now they are still alive.”

“I will have to treat the mass,” he says absentmindedly.

Turning back to the tobacconist, he says, “When you touch it, the more that you feel it, the more that it feels you. That is the better part of how it makes its mark.”

He gives the man a very grave look.

“I will not yet call you fortunate, though you may so prove to be. The sensible course upon stumbling into a scene as unnatural as this would have been to not pry further, and to have let the corpse alone. You may be relieved to hear that it is unlikely you were marked.  Nevertheless, we should not be too dismissive of the danger when the cost to confirm it is not dear.”

“I must stay here a time and treat the mass.” He gestures at the material in the corner.

“I will return to my rooms and then to your shop by nightfall. In the meantime, you should not stray far from your place unless it is a dire emergency. If by some action of the enemy I do not return to you, here is what you should do.”

“For this night and the following two, sleep with a slight crack in your casement, enough so that you can smell the air of the street. You must also have a flame within the room. A hearth is best but a lamp will do if it can be trusted not to go out while you sleep. If you ever wake in the night and smell this particular scent - remember it well - you must rise immediately and go to your flame.  Build a roaring fire. Do it in a hearth or in a bowl or burn your own furniture if you have to, but keep a sizable flame beside you. If the thing comes, it will come fast, and you will not have time to stoke a fire from embers. It may take a form that you find surprising, but know that it is a thing of darkness."

“Keep the flame between it and you. The thing cannot cross fire, though fire alone will not destroy it. Do not by any means go to sleep after you have first smelled the scent. When morning comes, send for me. You may ask for Renault at The Heron in Clerk’s Ward. If after three nights, no such thing occurs, then you are safe. At least as safe as any mortal in Sigil.”

“Anyways, I must attend to my business here. You may return to your shop. Thank you for your help.”

He reaches into his purse pouch and tosses the man one of the large silver coins.

The tobacconist catches it easily and stares at it for a few seconds.

“You’re saying it never touched me, though?”

He sighs. 

“Are you telling me that you touched the mass?”

“Not by intent, surely.  It was… not a simple matter to know what might be you and what might be it.”

“Then we must both hope that nothing delays me in discharging my duties before I attend you.”

“Aye”, he says.  “I catch the meaning.”

He remains stationary, peering intently at the blackened wall.

Then The Nameless One will promptly drop to his knees and employ the spike end of his axe on the paving stones. With painstaking, dragging strokes, he inscribes a semicircular region that fully encloses the gore and blackness. He is a powerful wizard engaged in a task to thwart his sworn enemy, and he cannot spare more than an occasional glance at the civilian standing by.

Within a minute, the tobacconist departs from view.

He watches out of the corner of his eye, and he continues his work calmly. That whole rigmarole should limit the man from taking further positive action today.

It occurs to him that the tobacconist never expressed any apprehension as to whether 'his man' or the street boy had themselves been marked by the evil creature. The thought brings a half smile to his lips. It is the smile of a magician catching a sloppy card palm in a rival.

There was some sort of an act going on there, yes, but his best instincts were not able to detect falsehood on the vital question: Did the tobacconist find his journal? 

Reflecting on his earlier chain of reasoning, the mystery he set out to solve is why the amnesia-inducing-adversary did not further constrain him after winning a decisive victory by slaying his body and inducing the amnesia. If the adversary wanted him delivered to the mortuary as part of a grander design, why present any useful information to the tobacconist at all? Why not act by proxy? Hmm. Could the street boy have been in league with the adversary? That should be less plausible even than the tobacconist, given that a boy could be expected to be less capable at concealing knowledge, should the amnesiac ever return to interrogate them. Though the boy could themselves have been the adversary in disguise. But... if that were the case then the adversary would have no reason to remain discoverable as that particular boy. Any way he looks at it, he'll have to glean what he can from what remains of this scene.

The scent around him is oppressive. It keeps triggering some flinch response among his inner mind’s personalities.