There's only somewhere between 4 and 7 of these stories. 3 are debatable (two with unnamed narrators, one with a Michael Kirowan), and I'm puzzled enough by the "Is John Kirowan the narrator of The Black Stone?" question that I'm going to the trouble of rereading the stories we know for sure that he's in for comparison to the three that are iffy. Now, I make no promises about blogging every step of this quest, but I figure it might help to put what I've gleaned from the first story out on the web in the hopes that other Howard fans might find it and
The plot is pretty simple. You know the saying that a friend will help you move, but a good friend will help you move a body? Well, John Conrad is a good enough friend that he will move your body in the precise manner laid out in a letter you give to him ten years prior to your death, as well as perform the creepy just-before-dawn occult ritual you describe in that letter. John Kirowan is an even better friend, because he will help Conrad do this at oh-god-its-fucking-dark-thirty in the morning at your creepy fucking house even though he's really scared and doesn't seem to like you very much.
The characters--despite serving the exact same story role if paired up with O'Donnel in later stories--contrast and work pretty well together in this story. The story starts with Conrad waking Kirowan up in the middle of the fucking night, agitated and upset. Right away, we have Conrad as the guy who finds trouble and Kirowan as the guy he goes to when he's realized there might be trouble. In the first conversation this gets confirmed because Conrad has an interest in the occult, the really dark and crazy parts of it, so he seeks out people like John Grimlan and befriends them. Kirowan doesn't seem to have much of an interest in the occult--beyond happening to know an awful lot about it for a guy who claims that "such talk is foolish" and there's a hoax going on--but he is loyal enough to do stupid things for the sake of another loyal and stupid man.
You get the impression early on that--even with a friend he's willing to help with dark occult rituals--Kirowan is putting up a logical exterior to cover the mentality of a believer. He denounces the supernatural as ridiculous even after he gives the exposition, but gets increasingly anxious about what Conrad's asking him to do even before the asking is over. He gets so tense he loses his composure more than one, yelling at Conrad, or just plain shouting in frustration. Throughout the story he is scared shitless, but he denies anything supernatural loudly and forcefully until the shit hits the fan and there is just no other explanation for it.
But--unlike rational Lovecraft heroes--Kirowan starts out either half-believing in--or more likely fully believing in and just outwardly lying about--this stuff to begin with so the discovery that there's something to it doesn't utterly break his brain. It just scares him and his stupid, loyal friend (I swear, Conrad is walking awesome in Dwellers in the Tomb but here he's just so damned dumb--his saving grace is that he KNOWS he's doing something really dumb) out of the house.
The story's set in 1930, and I believe it's meant to be while both these men are relatively young. In the other stories these are very brave and experienced men. But the setup for this is that Conrad goes through with something that is very obviously designed cause a potentially world-shattering disaster, a mistake I don't see the Dwellers in the Tomb version of the character making. Kirowan seems a bit smarter, going along with this, but he seems utterly unshakable in Haunter of the Ring and here he is definitely very shakable. This may be chalked up to O'Donnel narrating the other two stories. Through his eyes these men are practically made of iron, but this is a story where they can be a bit more human. Kirowan narrates, and he might just Conrad well enough to be able to tell he's really freaked out and made a huge mistake.
But even with that in mind, this story seems like a story about courage failing. Conrad sets up the ritual but needs moral support to go through with it--he establishes through his own dialogue that this is behavior caused by fear and not intelligence. ("It was understood, I suppose," he whispered, "that I should go through with this ghastly matter alone; but I had not the moral courage, and now I'm glad I had not.") When the Fourth Character--"the Oriental" as Kirowan calls him (I'm not going to, call me a white liberal baby for it if you want) because he remembers yellow eyes and a yellow robe and a vague impression of Asianness but can't actually remember what the guy looks like--arrives and takes over, they are both frightened into submission by his mere presence. In the end, they run screaming from the house.
Don't get me wrong, these characters are far from cowardly. On the flip side of moral courage, Conrad keeps a promise that could very well have gotten him torn to pieces by demons. Kirowan braves the same fate for Conrad's sake. There is serious facing of death and uncertainty and the unspeakable, just there isn't much punching of death and uncertainty and the unspeakable. I don't even mean literal punching, just the attitude that you're willing to punch death in the face. That attitude seemed to be there in some of the other stories, it's not here. I have to conclude this was pretty early on for them.
Aside from that, I get the impression Kirowan may be a little psychic. He's having nightmares when Conrad finds him and he gets nauseous when he sees the Fourth Character. That might be an idea Howard threw away, or just horror writing atmosphere. He is especially perceptive, though, he hears the monster not long after they enter the house, he notices things like the pattern on the robes, and he is pretty adept at reading Conrad's moods. I suspect Conrad considers him the friend who knows what to say to him when he's worried, and that's why he came to him to begin with.