You might want to read a thread on Dragonmount called "Equal yet not" from page 6 and onwards. I broke a lance for it there.

In a nuttshell;
I think it's how Elan Morin "betrayed all hope". He utterly gave himself away to Shai'tan. As soon as the possibility arose (back when pure chance spawned the first Myrddraal in the War of Shadow), I think Ishamael was able to link to a Fade.

I think that's why Ishamael is called Ba'alzamon by the Trollocs during the War of Shadows (see the note that was written only 200 years after the breaking from Rosel of Esam (sp?)). After all... Trollocs give names to Myrddraal. The funny thing is... the only Trolloc names we ever see are "Narg" (a true gutteral Trolloc name) and "Ba'alzamon" (an old tongue name back when Trollocs were still developing (deteriorating might be a more fitting term thoug) their own ..errmm.. language.

I think it's the secret Rosel of Esam hid in her High Chant poem that survived the breaking;
"Heart of the Dark. Ba’alzamon. Name hidden within name shrouded by name. Secret buried within secret cloaked by secret. Betrayer of Hope. Ishamael betrays all hope..."
To me, the 'shrouded by name' and 'cloaked by secret' refer to the Myrddraal part of what Ba'alzamon truly is.

There are also hints in how the Nae'blis his position is described: "but half a step below the Great Lord". Not three steps. Not one. Nope sir: HALF a step.
If anyone of the Chosen would realise what that entails, they'd all be running for the hills instead of competing for the Nae'blis position.
But we all know that Shai'tan does not share. He's the uber-control freak. The only one Shai'tan makes Nae'blis is as much of himself as he can this side of Creation.

At the start of the War of Shadows, that wasn't much, so the link to Ishamael is hardly perfect. But after some testing (aka Slayer; 2 bodies/ 2 personalities/ 1 living soul) and after having more power and after millenia of TP use by Ishamael (I think that means there is less of creation remeaning in the person), that link could be perfected when Ishamael needed transmigration.

Enter Taims body + a fade body.
One did live and one 'neverborn'.
Strip Taim of soul and mind.
Tranmigrate whats left of Ishamaels living mind (after millenia of TP use) into Taim's body and what's left of his dark soul into Taims body.
Link that to a Myrddraal body in the same way that was done with the experminent we lovingly call Slayer.
What do you get?

The perfect Nae'Blis. A Nae'Blis that is as close as Shai'tan can get this side of Creation (being the dark counterpart of creation). It's the only Myrddraal with a truly living mind and that's why this one can channel. Per Moraines words in TEoTW you need a living mind to be able to channel. RJ didn't put this in the text for nothing!

Well.. there is more. Lot's more. Like how Shaidar Haran almost misspoke when he was talking to the gars. That was when the new and improved Nae'blis was still comming to terms with what he now was and almost said he would give aran'gar to the Myrddraal instead of 'to my brothers'.

Like I said...read "Equal yet not" over at Dmount. And even that isn't the whole of it.

Cheerio,
Mik

Edit: added link (for a good laugh or some serious insspiration. )