Isam, Luc, Slayer
In The Shadow Rising, Slayer disappears near the Tower of Ghenjei while being chased by Perrin in Tel'aran'rhiod, leading to speculation that he can enter it. Readers have also theorized that he is somehow connected to Darkhounds, maybe as an evil wolfbrother. This is from the Dark Prophecy in The Great Hunt that describes Luc and Isam meeting and combining.
Evidence suggests that Slayer killed Janduin, Rand's biological father, when Janduin came to the Blight, as Janduin is said to have been in the Blight and come across a man who so resembled Tigraine that he did not defend himself against this man, suggesting his killer was in fact Luc/Slayer. He unsuccessfully attempted to kill Padan Fain in the Two Rivers, and as of Knife of Dreams is still hunting Fain. He also tried to assassinate Rand and Min.
In Winter's Heart Slayer assassinates two people using poisoned daggers at the command of an unseen man who he believes to be one of the Chosen. He also speaks of killing two Aes Sedai in the stone of Tear. He uses Tel'aran'rhiod to appear and disappear without warning and speaks of being granted many 'gifts'.
As of The Gathering Storm, Rand is unaware of Slayer's nature and relation to him. Birgitte states that Slayer is not just a new occurrence, but perhaps an old evil.
Perrin again encounters Slayer in Towers of Midnight where he placed the dreamspike to prevent Perrin's forces from escaping the Trolloc army.
Read more about this topic: Minor Wheel Of Time Characters