I haven't read, and don't own, this novel so any help I can give is going to be limited by that.
Based on the summary given in the other thread, though, here's my opinion:
In Planescape, the Blood War is run by the Dark Eight, a group of eight named pit fiends each responsible for a different aspect of the war effort. The project depicted in Dark Knight of Karameikos
would fall under the auspices of Furcas
, who is head of the Ministry of Mortal Relations.
From the Hellbound: The Blood War
Hellbound: The Blood War wrote:Furcas is in charge of Mortal Relations—or, in other words, the corruption and recruitment of willing primes and planars for the baatezu cause. He commands the erinyes and others who can walk the Prime Material Plane. Though Furcas scorns mortals and their petty ambitions, he's smart enough to see them as valuable tools, and, of course, potential recruits when they die. He sends his minions to lure the mightiest sorcerers and strongest warriors into the service of the baatezu. Furcas also seeks out those with potential for such '"greatness," and he's always on the lookout for berks who serve the cause of chaos (in order to do away with the dogs before they gain too much power).
The position of Dark Knight doesn't seem to be exactly "willing," but this still seems to be within Furcas
's purview of the corruption of mortals to further the baatezu cause.
In addition, Fiendish Codex II: Tyrants of the Nine Hells
states that the original Furcas
has only recently died and been replaced by a new pit fiend, given the same name to preserve the illusion of continuity (in Hellbound: The Blood War
, the original Furcas
was still alive, so this must have been very
recent). Killing off an ancient, canny villain like Furcas
off screen was something that annoyed me about Fiendish Codex II
, but I feel better about it if he dies at the climax of a novel. At least this way someone got a story out of it. Fiendish Codex II
doesn't specify how the original Furcas
The pit fiend in Dark Knight of Karameikos
is clearly high-ranking, since he oversees activities on countless worlds, but whether or not he's actually Furcas
or a subordinate depends on whether he's only
in charge of the Dark Knight project or if this is only one of the projects he's overseeing. I'm sure there are lesser pit fiends and other devils under Furcas
's command who oversee specific projects.
If the Dark Knight's suit of armor is black, it's probably made from arjale (described in "Nine Hells Revisited" by Ed Greenwood in Dragon #91). Arjale is a rare infernal metal often used to forge diabolic talismans. Alloyed with iron, it becomes a greenish-gray metal that's probably supposed to be the same thing as the Baatorian greensteel detailed in Planescape and 3rd edition.
In the Immortal Set/Wrath of the Immortals/Mystara cosmology, the Immortal most interested in mortal souls is Hel, so the Dark Knight project may ultimately be her brainchild. However, the project serves the Sphere of Entropy as a whole rather than any one Immortal in particular and is probably active whether Hel, Nyx, or Thanatos currently rules the Sphere.