I think the Dark One lied to Demandred about Asmodean having died the Final Death. He is the Father of Lies, and resurected the treacherous Lanfear who wanted to overthrow him and the Creator.
Moghidien commited sins equal to those of Asmodean and ended up mindtrapped.
I think Asmodean may have been put in one as well.
I know the black cords, Rand cut Ishamael's and we know he came back.
Just a theory...since there is a possible male Forsaken running around. Identities on who it could be have been presented, one of the female forsaken posing as a man.
Asmodean was revealed in his own thoughts to be loyal to the Dark One, but was afraid of paying the price of failure. And who'd have a personal grudge against Rand? Demandred, but the Trolloc attack in KoD seemed personal, and Moridin may prove unreliable if dealing with Rand requires personal sacrifice.
What better way to deal with it then an agent that Mordin is oblivious of. The same arguments could be Be'lal, but its debatable as well.
Just an idea. Flame away!