Well like Mrh said, Ishamael is actually Ba'alzamon.
[SIDE NOTE]
He is a lunatic but I don't think he thought he was the Dark One though.
In the Glossary for the first 3 books it says -
"Ba'alzamon - In the Trolloc tongue, "Heart of the Dark". Believed to be the Trolloc name for the Dark One"
Key word, "believed". The people/scholars of the world "believed" it, but that doesn't mean it's true.
It changes in the Glossary for Book 4 -
"Believed by most, erroneously, to be the Trolloc name for the Dark One."
So Ba'alzamon is actually Trolloc for Ishamael.
That kind of thing happens throughout the books, playing into the "time/retelling distorts info" theme and I love that.
Interestingly the "caverns of fire" in his eyes and mouth are very advanced side effects (called "Sa'a") caused by the long term channelling of the True Power (Dark One's power) which is
extremely addictive.
[/SIDE NOTE]
Now what I actually wanted to say.
Ishy wasn't properly Sealed into the Bore like the rest of the Forsaken were. He had periods where he (or at least his soul) was free to roam the world. I think the cycle was 40years out - 1,000yrs in.
That's why in the Prologue for book 1 he appeared to Lews Therin and why he was free long before the other Forsaken in Book 1.
He's responsible for the Trolloc Wars and the War of the Hundred Years.
He corrupted Artur Hawkwing, causing him to lay seige to Tar Valon and send his armies across the ocean to conquer distant lands.
The Dark Ones best weapon, and probably the only one of the Forsaken who actually understood what the Dark One would actually do if he broke free.
Cool huh?
Who killed asmodean Slayer?
No one's totally sure, RJ thought he left enough clues but he was surprised when so much debate sprang up around it
t:
Graendal is a good bet, but Sanderson said he'd include the info in the next books.
The forsaken who are brought back to life one is lanfear who is the guy is it the one who thought he was the dark lord or Asmodean?
It's not Asmo..... but I'm not sure if it's Ishy you're talking about either....