I am pretty sure I know where, but not who, Demandred is. In Shadows Sleeping (TDR) Perrin has a dream where he sees three scenes and four people. The first is a man who offers him wine and the chance to be a blacksmith again, Ishamael. He then sees a woman in white, Lanfear.

The last part is the important one finally three men having a meeting. One's Ishmael. The second to arrive has white hair and wears a Tarien coat; no doubt about it he's Be'lal.

The third man is described as “tall and dark and slender,” He has dark hair streaked with grey, a dark green coat, and boots with golden fringe encircling them. The fringe is the important thing, boots fringed with gold or silver is an Illianer fashion.

This man is definitely a male forsaken but can't be Sammael even though he was in Illian at the time, he had blonde hair and was definitely not tall (BWB). Since it's not Sammael it has to be someone else.

Rhavin, Aginor, Bethamel, Be'lal and Ishamael can be discounted. Two of them are dead, Be'lal and Ishmael are already present and according to Alteima in Fanning the Sparks (TFoH) Rhavin had “massive sholders and a deep chest” the man in the dream was slender.

By process of elimination it has to be either Asmodean or Demandred they are the only remaining male forsaken and both can fit the description.

Asmodean is described by Mat in Misdirections and Imre Stand (TSR) as both “a dark haired man in his middle years” and “taller than most, somewhere in his middle years.”

Demandred has dark hair and is almost as tall as Lews Therin was. In Decisions (TSR) Lanfear tells Rand that he is Lews Therin though “physically nothing is the same except your height,” therefore Demandred is almost as tall as Rand. Demandred is about the same age as Lews Therin who in Dragonmount (TEotW) described himself as just into his middle years.

Choosing between these two is the difficult bit. I have found two points suggesting that it is not Asmodean.

1) Perrin described the man in the dream as tall. Asmodean is described as taller than most by Mat, who is shorter than Perrin. Asmodean is above average but not necessarily possible to describe simply as tall, the height indicates Demandred.

2) In Pale Shadows Asmodean mentioned a saying he came across in Arad Doman (what he was doing there is anyones guess) and its pretty hard to be further away from Illian than Arad Doman while still being in the Wetlands.

These are fairly shaky evidence but it does indicate that Asmodean was not in the dream or Illian.

Conclusion: Demandred is in Illian. The fringed boots are only worn by lords therefore he is disguised as an Illianer lord, which one I don't know, but this means he can't be Taim, Dorobaine, Banshere or Masema.