From the journals of Mr. Eck:
I remain unconvinced that Jack and the entity we encountered at the church are one and the same. In all of his correspondences, Jack was identified, even characterized, by two elements: first, his uneducated, underclass manner of writing; and second, his obsession with whores. Even Chris worked hard to incorporate those elements into his ripper letter to Edwards to make it more ``authentic'' (I'm not bringing this up as an attempt to read Chris' mind, but to point out how important those two elements are). The ``entity'' on the other hand, demonstrated neither of these fundamental characteristics. Furthermore, there was (I believe) no evidence of a ritual orientation or canabalism in Jack's murders, whereas the entity demonstrated both during his killing of Polly. Now, if I were an upperclass gentleman who had some reason to be killing prostitutes, I suppose that I would cultivate Jack's lack of education and misogynist inclinations in order to throw off the police, but why would a semi-corporeal superhuman entity able to walk through walls and perform telekinetic marvels worry about throwing off the police? The fact that both Jack and the Entity seemed to be after Polly may be (I hate to say it) a coincidence, since Violet's group of friends may have appealed to Jack as a misogynist (they are, one could say, the perfect example of upitty women bent on overturning the morality of the status quo by pursuing education and equality--worse than prostitutes, some Victorian pub-goers might say).
If this is so, then Jack and the Entity represent not ONE card, but TWO (presumably the Black King and the Beast).
One more option, suggested by Jim: perhaps the Entity represents the astral (spiritual, virtual, whatever) form of the creature that called Polly to one of the nodes, so that it would be able to manifest itself in all its power and glory. Jack may just be a pawn, perhaps a schizophrenic who is susceptable to being possessed by the Entity. At the Entity's behest, Jack committs his murders, but does so within the context of his own sick little world. To control Jack, the entity simply turns the thumb screws on the poor man's mania--that would explain the low-class language and misogynism of Jack in comparison to the Entity's upperclass eloquence and ritualistic concerns. If this is so, we would appear to be back to ONE card--I should think the beast, since we know that the Black King was originally one of Lorenz' projects, and the way in which the entity seemed to entice Polly from alternate London sounds far more sinister and arcane than the sort of parlor tricks that Lorenz would create. (Chris, one clue to this might be in Lorenz' notes--he referred to the Entity as ``IT,'' as in ``if she gets to one of the nodes, she'll bring it down''; is the Black King in Lorenz' notes described in gendered terms, ``HIM,'' or as ``IT?'')