I haven't read the Prologue, but from the sound of it, it seems Demandred's role is continuing to be set up as hugely significant.

In fact, on Dragonmount I see some credible speculation on Demandred possibly even getting rid of Moridin in this book.

Given that Sanderson has previously emphasized that Demandred's secret will have a huge payoff in the last book, and the fact that the Prologue is only increasing the emphasis on his role, his contribution will have to be pretty much earth shattering once it is finally unveiled.

The worst is that despite some us having been reading this series for 20 years, and having reread all the books multiple times and analysed, speculated and theorized about it for more than a decade on various internet forums - and outlived some of those forums (Wotism, Wotmania and currenlty ReadandFindout in my case), we still don't really have any clue - even at this late stage - what Demandred has been up to.

It really is astounding, if you think about it.