Asmodean is not dead. Instead he was captured in Moridins second mindtrap (I don't understand how mindtraps work, do you really capture a person inside the box or do you only control their minds?). Remember that Moridin means death and I'm sure there must be some way Asmodean recognises Moridin. Maybe Moridin is Ishamael born into a new body like Balthamel and Aginor and therefore Asmodean could recognise him in some way. Who knows what the forsaken can and can't do?