I think Graendal is probably one of the most dangerous of the Forsaken - mainly because it is easy to dismiss her as vain and superficial.
Remember, she underwent a radical personality change when she swore to follow the DO - from plain jane ascetic to flamboyant, flashy pleasure seeker. She craves power (all of her "pets" are people who held positions of power). She wants to be Nae'blis because of the power it would give her. She has pulled strings, manipulated others of the Forsaken like chess pieces on a board - hiding her ambitions in plain sight, if you will, by pretending to be exactly what they think she is.
I can't remember the timing exactly but... she had her own private meeting w/ the DO where she was led to believe that she could, very well, be named Nae'blis. I think she must have done something that the DO would want to reward her for - like kill the one who was helping to hone Rand's skills. Asmodean was, by act if not by honest intent, betraying the DO's cause.
I think that Graendal killed Asmodean, went to Shayol Ghul and was rewarded with a "promise". The next Forsaken - Demandred, I think - to go to Shayol Ghul was told that Asmodean was dead, and with details enough to indicate that the killer had told of the deed him/herself.
And another thing... RJ incorporated so much of mythology and the like into his story. "Graendal" is very close to "Grendal". No, Grendal wasn't female - though, the argument could be made that his mother was, at least, as dangerous as Grendal was. Grendal was the thing most feared by all men. It's a play on words/names that I can see RJ utilizing...
Kind of thin, maybe, but that's my theory.
