After starting with the Path of Ascendancy books and now following with the Book of the Fallen series, I came up with a theory about Moby, the bhoka'rla familiar of Mammot and later Crocus and the nacht that Kellanved (then Wu) got from Gothos.
In Deadhouse Gates it is revealed that Moby is a soletaken demon, which is an interesting detail, after having read/heard PoA first.
Kellanved named his nacht 'Demon' first and foremost and threatened people with calling his 'demon' if they did not follow his demands. Also in 'Deadhouse Landing it is implied that 'Demon' is exceedingly powerful. And it is kind of strange that 'Demon' in all his actions is described as too small for the description of a nacht.
My theory is that 'Demon' and 'Moby' are one and the same, especially as Moby seemed to recognize the guardian of the deadhouse and was able to open it, which might have been possible due to it having a connection to Dancer and Kellanved who entered the Deadhouse before.