To add to, and perhaps restate in a clearer manner, what kaatunut has said:
Lews Therin Telamon was one of the strongest male channelers of his time,
perhaps equaled by only Ishamael. Howerer, it is actually stated in the books
that about a dozen Forsaken were killed during the War of Power, so perhaps
there were stronger. But LTT was definitely the most powerful male channeler
for the forces of the Light. LTT did love Lanfear; they were lovers for
a time, but he left her because he was disgusted by her lust for power. I
could go on for hours about Lanfear, as she is such a mysterious character,
rivaling even Ishamael. But in any event, LTT left Lanfear for Ilyena,
but Lanfear still considers him hers, and is insanely jealous of him.
Witness her actions on the docks when she finds out Rand slept with Aviendha;
she went psycho!
Lews led the Hurdred Companions to Shayol Ghul to seal the Bore. In
the books, the Bore is always referred to as a metaphorical hole; a thinning
of the pattern allowing the world to be closer to the Dark One's prison
at that one location. This is very ambiguous, as the Bore was physically
made, and by Lanfear at that. She was searching for a way to unite saidar
and saidin, but ended up freeing the Dark One. There is some speculation
that she had already turned to the Dark, and knew what she was doing, but
we have no idea. The books actually describe the Bore being opened. When Rand
is in the glass columns in Rhuidean, one of his Aiel ancestors witnesses
the opening of the Bore. But this is all an aside; we don't know the exact
nature of the Bore, or what exactly LTT did to seal it, but we know
it was sealed, and seven seals were created.
There is interesting speculation, mostly by Ishamael (ever the philosopher)
that LTT himself is merely the rebirth of the soul known as the Dragon,
which would explain how he came to have a creature no one has seen or heard
of on his banner. Ishamael speculated that Lanfear, or Mierin as she
was known then, didn't really open the Dark One's prison; it had been opened
long before, and sequentially sealed by an earlier rebirth of the Dragon.
The Wheel spun out another seven ages, and Mierin was the one that happened
to open it this time around. This would add some ammo to the camp that believes
Mierin had already turned to the Dark when she opened the Bore. If the
Bore had been opened countless times before, in earlier spinnings of the Wheel,
then the Dark One would be aware of it, and would be trying to influence people
in this age to reopen it, so he could break out forever this time. Well,
needless to say, it didn't work, and LTT sealed the Bore. Whether or not
he was the first to do so is a matter of serious debate.
The Hundred Companions consisted of all males due to disention among the
Aes Sedai of the time. LTT was the Tamyrlin, which was similar to the
Amyrlin Seat of this age, only that the Tamyrlin resided over male and female
Aes Sedai. For all that, he was unable to achieve any semblance of consensus,
so he led the hundred strongest male Aes Sedai against the Shadow, and drove
his forces to Shayol Ghul, the seat of the Shadow's power, and somehow
did something that we call the sealing of the Bore. The Forsaken,
or what was left of them, were also sealed in somehow, at least all of them
except Ishamael. Ever a mystery, many suspect that Ishamael was paritaly
sealed, or even never sealed. The Dark One tainted saidin, perhaps with
the True Power, perhaps not; we're not really sure. All we know is that this
counterstroke left saidin tainted, and led to the madness of all male channelers,
and the Breaking of the World. LTT and the other male chanellers didn't
die from channeling before this because there was no reason for them to; saidin
wasn't tainted. It was as pure as saidar, and as safe to draw upon. The madness
and rotting only came as a result of the taint. This is made apparent as
early as the Eye of the World, when Aginor is all scared and rotted, but
when he begins to draw on the pool of untainted saidin stored in the
Eye, he begins to heal.
The Breaking of the World was more than just the flattening of a few mountains,
and the moving of some seas. It really fscked stuff up. The land was entirely
changed, as the Forsaken have mentioned countless times in the books, when
lamenting the loss of their Age of Legends, and generally bitching about
how nothing is the same, and how put upon they are surrounded by primitives,
blah blah blah. All this has not stoped them from being killed like gimps,
but that's another story.
Lews finally went insane, and killed all of his blood, including his beloved
Ilyena, earning the name Kinslayer. Shortly after LTT is done killing
all his family, Ishamael appears in the flesh. This would seem odd,
seeing as all the Forsaken were bound with the Dark One at Shayol Ghul.
Guess not. Ishamael is at least partially unbound, if not completely. He
sees that LTT is completely mad, so he heals him, so that the enormity of
his situation can hit him like a ton of bricks. Ishamael does this using
the True Power, not the male half of the One Power. This is evident
when Ishamael says "I serve another power now." He is then surrounded
by darkness, and heals LTT in what he describes as a "different, more
painful" way. Ishamael is the only one that actually draws upon the True
Power regularly, as evidenced by the number of saa he displays. LTT,
fully healed, and sane at last, realizes what he has done, and becomes half
mad with grief. He Travels somewhere, and draws more of the One Power than
he can handle, resulting in a spectacular display of flames (not balefire),
and the creation of Dragonmount, and the island that would become Tar Valon.
The next Dragon Reborn, Rand, would then be born on the slopes of Dragonmount,
the site of the last Dragon's death, fufilling the Prophecy of the Dragon.