Isam is Lan's blood cousin. Isam is first mentioned late in TEOTW. In [TEOTW: 47, More Tales of the Wheel, 595], Agelmar begins his tale of the history of the fall of Malkier. Briefly, it is revealed that al'Lan Mandragoran is the son of al'Akir and el'Leanna. Al'Akir had a brother, Lain Mandragoran, who was wed to Breyan. Lain and Breyan were parents to a child named Isam (see section 2.5.1 for the family tree).
Agelmar goes on to explain Breyan's jealousy and grief over her husband's death in the Blasted Lands, and her plot with Cowin Fairheart, hero and Darkfriend, to seize the throne for her son Isam. This plot failed, and Breyan fled south with her infant son Isam, and was overtaken by Trollocs. Their bodies were never recovered.
It was at this time that el'Leanna and al'Akir sent their own infant son, al'Lan, south to Fal Moran to safety. The Glossary of LOC places al'Lan's date of birth in 953 NE, and [Guide: 28, The Borderlands, 247] places both Lan's birth and the fall of Malkier in 953 NE. Since Isam was also an infant at this time, he was likely born no earlier than 951 NE. Thus we can surmise that these events took place no later than 956 NE.
Lord Luc is the brother of Tigraine, former Daughter-Heir of Andor, and thus Rand's blood uncle (see section 2.5.1 for the family tree). In [TEOTW: 34, The Last Village, 441] we learn that Luc is believed to have died in the Blight while ostensibly in training to become the First Prince of the Sword. Tigraine later vanished, before she'd properly assumed her throne.
Later, in [TSR: 34, He Who Comes With The Dawn, 392] we learn that Tigraine ran off to become a Maiden of the Spear with the Aiel at the directions of Gitara Moroso Sedai, some four years before Laman's Sin. The Glossary of TSR places the Aiel War, which began as a direct result of Laman's Sin, from 976 to 978 NE. Thus, Tigraine disappeared circa 972 NE. Tigraine joined the Aiel in the Waste, became known as Shaiel, fell in love with Janduin, and went on to conceive and bear Rand al'Thor (see section 2.4.10).
On the next page, we learn that Janduin, Rand's biological father, was killed on a venture to the Blasted Lands by a man who looked so like Shaiel (who was really Tigraine, Luc's sister) that Janduin would not raise his spear. This is almost certainly Lord Luc, and is in the third year of the Aiel War, 978 NE.
Finally, in [LOC: 16, Tellings of the Wheel, 277] we learn that Luc himself may have been sent into the Blight by Gitara Moroso Sedai. After his disappearance, a year before Tigraine's flight, or about 971 NE, rumors whispered that Gitara sent him to find fame, or fate, or the Dragon Reborn or the Last Battle. Given her connection with Tigraine's flight, it seems very likely that the rumors are true.
Thus the timeline as we know it is:
Slayer is some form of combination of Luc and Isam, joined in a sort of split personality. While Luc and Isam seem to have separate mental existences and identities, the wolves of the Unseen World think of the construct as Slayer because of his penchant for killing wolves in that place, so the total construct is often referred to as Slayer in discussion.
The first reference tying Luc and Isam together in any way comes from the Dark Prophecy, scrawled on the walls in Fal Dara after the Trolloc raid. The relevant stanza [TGH: 7, Blood Calls Blood, 89] says:
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.
The exact interpretation of this stanza is uncertain, but clearly, Isam survived the Trollocs as long as circa 971 NE, when Luc went north into the Blight. Curiously, Luc and Isam would have been roughly the same age, as well. There was evidently a confrontation; one died, one lived, but both still exist in some combination.
Now, the first time we encounter Luc or Isam in person, rather than as background, is in those segments of TSR set around Perrin's trip back home to the Two Rivers.
The middle-aged Lord Luc who arrives in the Two Rivers, claiming to help the villagers with the Whitecloaks and Trollocs, is that same Luc. His age and coloring are correct, and Perrin muses that if he resembles anyone, it is Rand. A cousinly resemblance, no doubt. Luc is Rand's blood uncle.
In [TSR: 28, To the Tower of Ghenjei, 320-325] Perrin has several encounters in the Unseen World. One is with a man who tries to kill him, a man with a cold, inhuman scent to Perrin's nose. Hopper later identifies this dangerous creature as "Slayer" after Slayer leads Perrin on a chase to the Tower of Ghenjei. Then Birgitte appears. She identifies the Tower, connects it with the Aelfinn and the Eelfinn, and warns Perrin away from it and from Slayer.
Later, Perrin sees Slayer in the Unseen World looking much like Lan, dressed and styled in the Malkieri fashion [TSR: 42, A Missing Leaf, 476]. He muses that the man looked enough like Lan to be a brother. This is Slayer as the Isam persona.
In [TSR: 53, The Price of a Departure, 614-615] Perrin faces Slayer in the Unseen World, and shoots him with an arrow. Slayer disappears from the Unseen World, and when Perrin wakes up, learns that Lord Luc had suddenly run off as if wounded.
Here, Perrin connects the two. He notes the simultaneity of the wounds, and notes the same icy, inhuman smell from both of them. Traveling to the Unseen World in the flesh is said to cause a loss of one's humanity. Both these effects are hints that Slayer not only accesses the Unseen World, but does so in the flesh.
In [TSR: 56, Goldeneyes, 645-646] the Trollocs in the Two Rivers form a battle cry out of the name Isam. This is our first indication that though his activities may seem to be limited to the Unseen World, the Isam persona of Slayer can exert influence on the real world.
We don't see Slayer again until WH. The information we gain there, though, has led to considerable revision of our earlier assumptions about Slayer's nature, abilities, and role in the series.
The TSR encounter led us to conclude that Luc and Isam were parts of a single being, aptly named Slayer by the wolves he kills in the Unseen World. We know from Egwene's training what some of the properties of the Unseen World are. Relevant properties here include the loss of one's humanity (as in, a cold, icy, inhuman scent coming from both Luc and Isam) after repeatedly going to the Unseen World in the flesh, and the Unseen World's tendency to reflect the traveler's mental state.
Furthermore, since in TSR we only saw Luc in the real world and Isam only in Tel'aran'rhiod, it seemed safe to assume that Luc was the "one who lived" in the Dark Prophecy stanza, and that the dead Isam was now somehow piloting Luc's body. The logical conclusion was that Slayer could only appear as Luc in the real world and only as Isam in Tel'aran'rhiod. But this doesn't jibe at all with what we see in WH.
The first time we see him, he's not specifically named, but it's pretty obvious that the man spying on the Supergirls in [WH: 10, A Plan Succeeds, 240-241] is Luc: "A man was standing there watching, a man as tall as an Aielman, with dark red hair faintly streaked with white, but his high-collared blue coat would never be worn by an Aiel"..."his hard face seemed somehow familiar"..."'Rand,' Egwene said. 'He could have been Rand's uncle.' Of course, Elayne thought. If Rand had a mean uncle." It doesn't get much clearer than that. What's important to note is that it was Luc the girls see in T'A'R, not Isam.
The second time we see Slayer in WH is his assassination attempt on Rand, in [WH: 22, Out of Thin Air, 447-449]. First we see Isam, in T'A'R, studying his victims-to-be; then: "...he carefully unsheathed the two poisoned daggers and stepped out of the Unseen World into the waking. As he did, he became Luc. It seemed appropriate". Note the last sentence. It was not necessary to become Luc - just appropriate. Further down the page he muses that he had been given "many gifts", though immunity to poison or weapons was not among them. Then he steps back into T'A'R as Luc, and only switches back to Isam after talking to his mystery employer (see section 1.4.4).
Well, that blows our previous theory right out of the water. This passage demonstrates that Slayer can appear as either Luc or Isam in the Unseen World; and strongly implies (though does not state) that he can appear as Luc or Isam in the world of the flesh as well (because he was Isam when he murdered Amico and Joiya - see below).
It's worth noting here that we only see Slayer actually switching from one man to the other in T'A'R, so it's still reasonable to assume that even if he can choose to be either Luc or Isam in the waking world, he can only change from one to the other in the Unseen World.
It also seems clear that both Luc and Isam's personalities have survived intact, and that Luc is just as evil as Isam. In fact, they seem to be quite the bosom buddies.
However, this new information still doesn't cast a lot of light on how this merge of two people came to be, and in fact muddies the issue even more. If Slayer is not a case of possession, then what the hell is he? How much of Luc's future did Gitara Moroso see when she sent Luc to his doom?
(As to the last question, Mike Edenfield offers: "Note that when Elaida has a Foretelling about the Last Battle, the most she can get out of it is that 'The royal line of Andor' is involved. Mayhaps Gitara's Foretelling didn't specify which sibling needed to be sent harrying off to wherever, so she sent them both.")