[John Novak, Leigh Butler]
In [WH: 22, Out of Thin Air, 448-449], Slayer, as Luc, uses poisoned daggers to kill a man and a woman, realizes they were not the ones he was after, and then steps into the Unseen World. There he meets his "patron of the moment", who tells him to wait before trying again and to say nothing about the matter. Luc thinks this is too bad, since he had been looking forward to killing "his nephew and the wench".
Clearly, this refers to Rand and Min (Luc is Rand's uncle), and just as clearly Slayer was ordered to kill Rand (secretly) at this patron's command. The question, then, is who's giving these orders?
Slayer's current master uses a strong disguise woven of the One Power which makes it absolutely impossible to see or hear him clearly enough for any identification. Slayer believes that his current master is a male, and that therefore he is a male Forsaken, as no other man who could channel would know him, know how to contact him, or dare to command him, though he notes that none of the Forsaken he had met had ever taken such precautions.
At that point, the surviving male Forsaken are Ishamael/Moridin, Aginor/Osan'gar, Balthamel/Aran'gar, and Demandred.
(Sammael was an early favorite for the identity of Slayer's boss, but RJ's assertion that he died in ACOS means we must reject him as a candidate (see section 1.1.1), though there is an outside possibility, I suppose, that it may be a reincarnated Sammael. There is no evidence to suggest that, though, and one would presume that if Sammy had been resurrected, Moridin would know about it and be busily reining Sammael in, just like all the other FS.)
It's unlikely that Moridin would have a motive to disguise himself so thoroughly. He's Nae'blis - who would he need to hide from? Especially since it is obviously common knowledge among the Forsaken that Moridin is already using Slayer to hunt Fain [WH: 13, Wonderful News, 316].
Demandred, on the other hand, has both a strong motive to kill Rand (personal hatred) and a good reason to want to disguise himself - to hide what he's doing from Moridin. Slayer, on Moridin's orders, is supposed to be doing something else right now (killing Fain), and commanding him to go after Rand would obviously interfere with that task [Fairfax McCandlish]. A point further in Demandred's favor is that he already had his own "kill Rand" plans set in motion in TPOD and WH (i.e. the Fearsome Foursome), so employing Slayer as a backup fits his M.O. In addition to this, Demandred's link with the Fab Four means he would have known where to tell Slayer to look for Rand; remember, up until Gedwyn, Torval, Rochaid, and Kisman put together all the clues Rand had left behind and followed him to Far Madding, none of the bad guys knew where Rand was.
Osan'gar, as Dashiva, took part in a previous attempt to kill Rand after Rand made known his intent to cleanse the Taint in TPOD (see section 1.4.10) so he has a motive to employ Slayer (and the same reason as Demandred to disguise himself). However, Osan'gar is terrified of Moridin [WH: 35, With The Choedan Kal, 646]. It's doubtful, therefore, that he would even attempt to commandeer one of Moridin's personal lackeys and risk discovery. Plus there's no reason to think he would know where to find Rand, anyway - Kisman's POV makes it clear that Dashiva had had no further contact with the Fab Four after the TPOD attack [WH: 22, Out of Thin Air, 441].
Aran'gar has no known motive, and is mentioned only because she might appear as a male in the Unseen World.
It is also possible that Slayer's current master is Mazrim Taim.
While Taim is not Demandred, he is most likely a Darkfriend (and possibly something more than that), which could plausibly provide him with knowledge of Slayer and how to contact him. His disguise would be motivated by preventing Slayer from knowing that someone other than a Forsaken is commanding him (not to mention keeping it secret from the Forsaken themselves). Taim certainly appears to be ambitious and ruthless enough to attempt such a subterfuge. Taim, like Demandred, also has his association with the renegade Asha'man in his favor (i.e. he could have known from them to tell Slayer to look in Far Madding).
Also, Slayer's comment about how non-Forsaken would not dare try to command him is too taunting. [Oleg Ozerov].
It is worth noting, though, that the chapter icon for "Out of Thin Air" is associated with the Forsaken (the snaky square), which suggests that Slayer's assumption is actually correct [Jean Dufresne]. But, Matt Hackell counters, "In the later books this connection has broken down far enough that it can only be corroborating evidence, not counterevidence, especially when another BIG BAD (such as Shaidar Haran, Fain, or Slayer) is in the chapter."
In conclusion, it seems that the two best candidates for Slayer's mystery employer are Demandred and Mazrim Taim.