so, idk if this hasn't been said but I have a theory.
SPOILER WARNING for Until the End and Seasons of War!!
So this is for phase three of Skulduggery Pleasant.
So you know all that crap that happened at the end of Until the End (the whole world restarting because of Darquesse and some other dude), would they have control over dimension x? If not, I think I know who the next villain is. So, it is implied that Dexter Vex killed Saracen in Seasons of War due to him being infected by a Draugr, well I don't think he did. He was going through immense amounts of grief already, and with the death of Ghastly and Anton and the stress he was under, he couldn't do it. So he left Saracen there, hoping he wouldn't be killed. And then they left, probably leaving a zombified Saracen. Now this is where my theory gets a bit far fetched, what if they remade Saracen from scratch due to the fact he is in another dimension but, when they do, og Saracen regains a small bit of humanity and realises that the others have left him to die, gets pissed off and somehow makes his way back to the original dimension.