How was Ishamael able to visit Lews Therin after the Dark One was sealed away?

Question

At the end of the War of Power, Lews Therin Telamon led the Companions to seal away the Dark One. By luck or design, they caught the entirety of the inner circle of the Forsaken there and sealed them away, too, shattering the forces of the Shadow.

How, then, was Ishamael still able to visit Lews Therin afterwards and grant him the moment of sanity and name him Kinslayer? How was he able to masquerade as the Dark One over the three thousand years between the War of Power and the present day? Is this ever explained?

Answer 1

According to wikia, the book The World of Robert Jordan's The Wheel of Time (which was not written by Robert Jordan himself, but based on detailed notes he supplied) contains an in-universe theory to explain this:

Aran son of Malan son of Senar (born roughly 50 AB) had a theory based on Ishamael being thrown out and touching the world for specific periods of time. Aran had heard claims that people had encountered him as long as forty years after the sealing of the Bore. Using lost manuscripts, Aran concluded it might have taken some time for Ishamael to be brought into the prison with the remainder of the Forsaken, and might possibly have been "thrown out" in a regular cycle.

Answer 2

I remember reading in the Wheel of Time books that:
MAJOR SPOILERS!

Ishamael was only partially caught when Lews Therin and the 100 companions sealed the Bore, which is why he constantly flickers from place to place. All of the other Forsaken where firmly sealed when they were called to the pit to meet with the Dark One.
The Dark One was able to launch a counter stroke at the time of the sealing and taint Saidin, the male half of the one power. The taint drove Lews Therin insane and he kills his own wife and family.
Ishy then pays him a visit, partially heals Lews Therin's insanity, so he can understand that he killed his own family, and then taunts him into killing himself.
So, for the next 3,000 years Ishy creates epic mayhem by impersonating the Dark One, starting the Trolloc Wars, starts the Black Ajah, convinces Artur Hawkwing to wage war against the Aes Sedai and send his sons across the ocean (i.e. Seanchan formation) et al.

Answer 3

I would have to disagree with part of djm's assessment. He said, "Ishy then pays him a visit, partially heals Lews Therin's insanity, so he can understand that he killed his own family, and then taunts him into killing himself." From several mentions in the series, there are only two things that cannot be Healed - death and insanity. Although there were indeed some Aes Sedai who specialized in diseases of the mind, there is no mention of the One Power being used to deal with these issues. If insanity were curable, the 100 Companions might never have gone insane and Break the World.

So, even if insanity were "partially" curable, that should have still been enough to keep the once benign male Aes Sedai from committing mass murder. I would imagine that most of them would choose to die rather than commit mass murder or lose the ability to channel, either one.

Answer 4

Ishmael used the True Power, the Dark Ones own power to make Lews Therin lucid enough to see what he did. The same power Moridin uses and the same power Rand used to break the Male A'Dam and kill Semihrage

Answer 5

Remember, the Dark One can reincarnate the "Chosen" and according to the recently published WoT Companion, a male channeler of his strength would theoretically live about 800 years, so he'd only be on his 3rd reincarnation when Rand is reborn. Also, the Bore wasn't perfectly sealed, and those closer to the "top" were still affected by time (remember Aginor and Balthamel's appearances at the end of TEotW), and finally, saidin-induced insanity is curable. In ToM, Nyneave heals more than one man who's going mad, and she says that when she Delves them it looks like Compulsion that was woven with the True Power.