First let's make it clear that Shaidar Haran is not your average Halfman.
1. He has a sense of humour. According to several of the experts on Shadowspawn (Osan'gar=Aginor, Graendal and Moghedien for example) Myrddraal are cold and dispassionate. 2. He can shut people out from the True Source. Osan'gar states that it's not an ordinary shield, but nevertheless it's not something Halfmen should not be able to do. 3. When Graendal was visited by Shaidar Haran ( tPoD, New alliances) "a ball of...something...appeared in the air, a dead black globe, but a silver light filled the room". Graendal is stunned! Again, not the average Myrddraal achievement. To me, it sounds like the True Power was used.
So we have a "funny" Myrddraal with MORE than average powers, who is possibly a TP user. The obvious question is: Where did Myrddraal version 2.0 come from? Well, we know that Halfmen were a side effect from the development of Trollocs by scientists (mainly Aginor) devoted to the DO during the AOL. There doesn't seem to be much research going on in Randland today and Aginor was killed as soon as he was released from his 3000-year imprisonment (only to be brought back as Osan'gar at the same time as Shaidar Haran enters the show). Moreover, the seals still hold back the DO from touching the world as much as he was able to during the AOL, so I don't think he could upgrade Myrddraal on his own. Conclusion: A soul of a very powerful human being was put in Shaidar Haran's body. What powerful souls does the DO have available then? The dead forsaken of course! Some might argue that it could be someone else from the AOL brought back to life, but if that was the case, why hasn't the DO resurrected a whole bunch of his followers from back then (cheap labour!)?
What do we know about Moridin then?
1. He seems to know all the forsaken very well, and he isn't impressed by any of them. 2. He is a serious TP addict. 3. He has been named Nae'blis (at least that's what Moghedien, Cyndane and Shaidar Haran says).
Who knows all the forsaken well, and who could channel the True Power? Another forsaken, and since Moghedien doesn't recognise him, a dead forsaken. And the candidates are:
1. Aginor Back as Osan'gar, who might be Dashiva. Anyway, he is more of a scientist than a leader. 2. Asmodean A betrayer back as Nae'blis?!? I think not! 3. Balthamel Back as Aran'gar, who is a woman. 4. Be'lal Dead by balefire, no turning back. 5. Ishamael ? 6. Lanfear Many people seems to think that she (if she died) has been resurrected as Cyndane, and I agree. 7. Rahvin Dead by balefire, no turning back. 8. Sammael Died (?) after Moridin appeared.
And the winner is ...Ishamael. After all, he was something of a leader for the Forsaken during the AOL, and he has been going on about him and the Dragon fighting each other a thousand times over during the Ages.
So, why do I think that Shaidar Haran and Moridin are the same person?. Let me quote (tPoD, New alliances) Shaidar Haran: "When I speak, you may consider that you hear the voice of the Great Lord of the Dark." ... "I am his hand in this world, Graendal. When you stand before me, you stand before him." After hearing that you get the impression nobody in this world could stand above him, but a couple of pages before Moridin was named Nae'blis, and Shaidar Haran doesn't disagree. The Nae'blis is supposedly the person who stands one step below the DO and commands all others. So do we have two people leading the dark forces suddenly?
Another strange thing: In aCoS (Mindtrap) Shaidar Haran (who was the first person to hold the mindtrap) tells Moghedien to close her eyes, then throws her out of her strange prison, and then another person tells her to look and suddenly she is in front of Moridin, who now has her mindtrap. I'm sure there's a thousand good reasons for why she had to close her eyes, but the one I favour is that she weren't allowed to see the Shaidar Haran to Moridin transformation.
Third piece of evidence: In the beginning of tPoD (Deceptive appearances) Moridin plays sha'rah. He states that only nine living persons remember this game. What nine persons from the AOL are alive at the moment that Moridin is aware of? Well, himself, Graendal, Moghedien, Demandred, Mesaana and Semirhage of course. That's six. Then we probably have Lanfear as well, either she didn't die or she's been brought back as Cyndane. Seven. Sammael might have survived his meeting with Rand or maybe been resurrected and Asmodean could also have been brought back (though I strongly doubt it). So, with a lot of ifs we end up at nine, but if Moridin knew about the 'gars, there would be nine persons without the need for Sammael and Asmodean. However, when Osan'gar and Aran'gar woke up Shaidar Haran said that "None but I and the Great Lord know you live." (LoC, prologue). So, did Shaidar Haran really tell Moridin this big secret?
After all that overwhelming evidence in favour of this theory, there's one more thing to prove to get some order and balance into WoT. Did Ishy also have a Myrddraal alter ego? Well, maybe. In the prologue of tDR Jaichim Carridin is visited by a strange Myrddraal that shows (a twisted) sense of humour and actually laughs. We know that Carridin gets his orders from Ba'alzamon = Ishamael, so the Halfman also has a connection to Ishamael. What kind of connection I suspect should be obvious by now...
To finish this story I would like to ramble a little about why the DO has given Moridin/Ishamael two bodies to use at whim. Well, for Moridin I think the answer is clear. His greatest difficulty as Nae'blis (except from winning the last battle) will be to get the other Forsaken to obey him. I mean, if he walked right up to Demandred and said "Hey, I'm Nae'blis. Do as I say." the answer would probably be "Yeah! Right!". Shaidar Haran, on the other hand, seems to scare the shit out of any Forsaken he meet. Why (if) Ishy got a Myrddraal alter ego from the beginning is something you can only speculate about. Maybe it was necessary to slip out of his prison more often. Anyway, I think that one of the other Forsaken somewhere described him as "less than human", and Ba'alzamon certainly didn't appear like your next door neighbour (if he did: move!). I guess that could be a result of heavy TP usage or being partly imprisoned with the DO for 3000 years. On the other hand, what would happen with a man if he every now and then turned into a Halfman for a long period of time...