Who is Slayer?
In a series renowned for it, Slayer represents both the best and worst of The Wheel of Time’s world-building. It’s the best because he’s a tremendous character with a real backstory that ties into the fundamental themes of an epic narrative. It’s the worst because the clues to his real identity are so sparsely scattered it’s hard to even know they’re pieces to the same puzzle.
That said, let’s answer the question.
Slayer is an Amalgam Created by The Dark One
He is the combination of two people, one alive and one dead. Their personalities are just mushed together, while he can switch back and forth between bodies at will. He can enter Tel’aran’rhiod at will, and in the flesh. He is both strong and experienced in the World of Dreams.
That makes him an excellent assassin. He can zip in, kill someone, and either zip out, or change his appearance. He is used for that purpose by the Forsaken and the Dark One. He kills Joia Byir and Amico Nagoyin in the Stone of tear. He kills two random people in Winter’s Heart after mistaking them for Rand and Min. He was sent to the Two Rivers to kill Padan Fain and shows up as Lord Luc.
He was assigned to help Graendal deal with Perrin, and in The Last Battle, he’s assigned to kill Rand. Perrin kills him first.
That’s the bare bones explanation, and Jordan could have left it at that, but he didn’t. He gave us the names of the people amalgamated into Slayer: Luc Mantear and Isam Mandragoran.
The Andoran Succession
While an Accepted, Elaida do Avriny a’Roihan had a foretelling showing that the Andoran royal family was going to be key to the Last Battle. Shortly afterwards, Queen Mordrellen Mantear died, the daughter-heir could not be found, and Andor was plunged into the Third Succession War, which was won by Morgase Trakand.
When Elaida saw Morgase was likely to win the war, she attached herself to Morgase, ultimately becoming her Aes Sedai advisor. But before Mordrellen’s death, she had an Aes Sedai advisor named Gitara Moroso.
Moroso is famous for having the foretelling that sent Moiraine and Siuan on the quest for the Dragon Reborn. She foretold that disaster would come to Andor and perhaps the world if Tigraine, the Daughter-Heir, didn’t disappear into the waste telling no one, become a Maiden, and not return until the Aiel went to Tar Valon.
Tigraine was married to Taringail Damodred, and had a son Galadried, but she left anyway, became a Maiden, fell in love with Janduin, and delivered the baby that would become Rand al’Thor on the slopes of Dragonmount amid the Blood Snow.
Tigraine had a brother, Luc Mantear. Gitara Moroso told him of a foretelling of disaster that would come to Andor if he didn’t abandon everything and go into the Blight. He went and died in the Blight before being merged with Isam Mandragoran.
As Slayer, in his Luc persona, he ran into Janduin and killed him when Janduin refused to raise his hand against someone who looked so much like his dead wife.
The Fall of Malkier
Isam Mandragoran is Lan’s cousin.
Lan’s father was the al’Akir, the last King of Malkier. He had a brother, Lain, who married a woman named Breyan. Lain was well-respected and honored, but not as much as al’Akir, and Breyan was jealous of the King.
Breyan goaded Lain into raising an army and invading the Blasted Lands with the goal of taking control over Shayol Ghul itself. Lain and his men were killed, and Breyan blamed the King, saying that if he’d taken the rest of his armies to help, Shayol Ghul would have fallen.
In her grief and anger, Breyan plotted with Cowin Gemallen to overthrow al’Akir and put her son Isam on the throne. They moved soldiers from the border forts along the Blight border to convene on Malkier city.
But Cowin Gemallen was a darkfriend and betrayed Malkier to the Dark One. With the border forts under-manned, Trollocs swept over them and through Malkier.
Al’Akir and his wife anointed their son Lan as battle lord and the next King of Malkier and sent him with the 20 best members of the king’s guard to Fal Moran. Then they set off to defend Malkier from the Trolloc invasion. As the Companion puts it, “They died at Herot’s crossing and Malkier died with them.”
During this desperate struggle, Breyan fled with Isam and neither were seen again, so we don’t know the precise story of how Isam was combined with Luc.
And This is in the Books?
Most of it isn’t unless you think “the books” includes the Companion. What does appear in the books is a bit of the Dark Prophecy from Fal Dara:
Luc came to the Mountains of Dhoom
Isam waited in the high passes.
The hunt is now begun. The Shadow’s hounds now course and kill.
One did live, and one did die, but both are.
The Time of Change has come.
Blood feeds blood.
Blood calls blood.
Blood is, and blood was, and blood shall ever be,
After she hears this, Moiraine thinks to herself that Isam is blood kin to Lan, and that she’ll have to keep this from him.
When the Trollocs attack Emond’s Field in the ending battle in The Shadow Rising, they shout “Isam” before they charge.
The prologue to A Memory of Light includes a sequence from Isam’s point of view that suggests he was raised in The Town, a sort of haven for Shadow-aligned people in the Blight. It’s in that POV that he’s assigned to kill Rand before Rand can fight the Dark One.
There are other snippets in the books here and there, but that’s the bulk of the discussion of how Luc and Isam combined to make Slayer.
If there’s anything else in the books you find confusing, let me know in the comments and I’ll see what I can do.