As far as I remember, there was a scene with him spying on Elayne in T'A'R in LoC.

And given his POVs during the Cleansing, he is probably the most capable and far-thinking of the Forsaken. Even Graendal acknowledges that.

To some extent, tGS's prologue actually softened Semirhage before the rest of the book through Graendal's POV, while simultaneously sharpening Demandred's claws - I think he is the one we'll learn to fear in ToM. So far we've seen his work in Masema and Asunawa, if not others (Weiramon perhaps, maybe also Taim).

His weakness is of course quite obvious - an unswerving hate for Rand/LTT. How this can be exploited however is another matter.

Also, Arang'ar seems to be a lingering threat. I think it's more likely that Narishma will deal with her than Demmy, given his anger over Eben Hopwil's death.