Why are we sure that Moridin is Ishamael returned from the grave?
In TGS, Moridin pulls Rand into the World of dreams. Rand immediately recognizes him as the man that saved him in Shadar Logoth and as Ishamael. [TGS 15: A Place to Begin ]So what has Moridin been using his new lease on life for?
The Watcher and the Wanderer
[Pam Korda, Paul Khangure]
In addition to Moridin's overt appearance in ACOS, there were two other appearances by mysterious men of the Dark persuasion in that book. First, we met the Watcher, i.e. the guy watching Graendal and Sammael in [ACOS: 20, Patterns Within Patterns, 356]. Then, we met Moridin in [ACOS: 25, Mindtrap, 417]. Finally, there is the "wanderer" that Rand met in Shadar Logoth during his fight with Sammael in [ACOS: 41, A Crown of Swords, 656]. Even before TPOD, it was reasonable to assume that all three were the same person. After all, how many mysterious True Power channelers do we need? Given that neither the "Watcher" nor the "Wanderer" appear as distinct individuals in TPOD, we have even more reason to believe that both are Moridin, who is Ishamael.Let us consider the specific bits of evidence.
The Watcher
We meet the Watcher in [ACOS, 20, Patterns within Patterns, 356-358]:
1. Obviously a Forsaken from the AOL. He knows about AOL technology - callboxes, Mask of Mirrors and fancloth, knows the Forsaken by name and appearance. He definitely knew Sammael, as opposed to knowing of him. Just a guy from the AOL is not enough; he has to be someone who interacted and got to know Sammael's strengths and weaknesses. Notice there was no fear of interfering with not just two of the Forsaken, but with a potential battle between them! Furthermore, he uses the phrase "so-called Aiel" which we have no reason to expect anybody except a relic from the AOL to use; the Forsaken are the only AOL relics in action, except Birgitte and "LTT."
2. Senses saidin being held by Sammael, skin prickles when Graendal channels. This indicates that he has the ability to channel saidin.
3. He uses the TP. Note that he does so even though he can use the OP. This indicates that he probably uses it more than your average Forsaken--he could have Traveled with the OP, but he used the TP instead. Note that by the time he left, Sam had left the vicinity, so there was no chance of him being detected.
4. He has eyebrows and eyes (i.e. he is not Shaidar Haran).
5. He has "expertise in many areas Sammael scorned. In some he favored, too."
6. He doesn't seem to have the same attitude as the "normal" Forsaken towards Nae'blis-ness. In [ACOS: 20, Patterns Within Patterns, 358], Sammael mentions (lies) to Graendal that he "will be Nae'blis." Graendal then stops arguing with Sam and follows him through his gateway. We then have: "The watcher smiled crookedly behind his fancloth skulker's mask. Nae'blis. That explained what had brought Graendal to heel, what had stayed her from killing Sammael. Even she would be blinded by that." It seems as if the Watcher is NOT blinded by the "Nae'blis" carrot, unlike the "normal" Forsaken.
7. Obviously, he has an interest in what Sammael and Graendal are up to, since he is spying on them.
What can we conclude from these observations? From 1 and 2, we can conclude that the Watcher is Moridin/Ishamael, Osan'gar/Aginor, or Demandred.
Aran'gar/Balthamel is out, because she thinks of herself as "she," as indicated in [TPOD: 16, Unexpected Absences, 332-333]. The relative lack of interest in Sammael's supposed Nae'blis-hood rules Demandred out. (Recall Demandred's reaction to "WOULD YOU BE NAE'BLIS?" in the LOC Prologue.) Osan'gar/Aginor/Dashiva is pretty much ruled out as well. Not only is he depicted as a terrible skulker (and a total wuss to boot) in WH, he thinks to himself about how "He had never been a soldier, not really. His talents, his genius, lay elsewhere" [WH: 35, With The Choedan Kal, 645]. This plus the description of Aginor's activities during the AOL (Shayol Ghul's resident Mad Scientist) is in direct conflict with the Watcher's thoughts about having "expertise in some [areas] Sammael favored." Aginor's only apparent realm of expertise seems to have been biology, which Sam wasn't really into.
The last, and overwhelmingly most likely, possibility is Moridin. This is indicated by the Watcher's use of the TP in a circumstance where the OP would have sufficed. Given the general attitude of the Forsaken towards the TP, it is unlikely that there are TWO such TP addicts around, and that Osan'gar is one of them. Unlike Moggy, Osan'gar didn't even consider using the True Power when he found himself shielded by Shaidar Haran. The fact that the Watcher was keeping tabs on Sammael and Graendal connect him to both the Wanderer (who interfered with Sammael and Rand's fight in SL-- something he could have only done if he knew what Sammy had been up to) and Moridin. We know Moridin was watching Sammael, because he thinks about how foolish Sam's plans are in [TPOD: 2, Unweaving, 81-82], and his minions Moggy and Cyndane seem to know about Graendal's connection with Sam when they visit her in [TPOD: 12, New Alliances, 262-268]. Thus, everything points to the Watcher being Moridin.
The Wanderer
We see the Wanderer in [ACOS: 41, A Crown of Swords, 656-659]:
1. Description: Big fellow. Has a deep voice. Little older than Rand. Black coat, black hair. Rand doesn't recognize him.
2. He's most likely not a Third Age person. Knows Sammael, including how Sammael thinks. He clearly knows Sam fairly well, and thus is probably from the AOL. Furthermore, he has "never been afraid of Aes Sedai." Everybody in Randland proper (i.e. not Seanchan, not Aiel, and not Sea Folk) grows up hearing stories of Aes Sedai like those the Emond's Fielders did. Such stories engender some sort of awe in the listener, yet this guy acts like AS are no problem, and has never thought otherwise. This comment is easily understandable if he's from the AOL-- back then, he WAS an Aes Sedai, and modern AS are but "untrained children" to him.
3. He uses the TP instead of the OP (balefire, and disappears without Rand sensing saidin or saidar). This is a situation where using the TP instead of the OP could be dangerous-- it might make Rand suspicious-- and yet the Wanderer uses it. This is evidence that the Wanderer is a TP addict.
4. Even apart from his TP use, he is clearly a Minion of Darkness. He calls Rand a fool, he doesn't "care to see [Rand] die today," he doesn't "intend to carry [Rand] on [his] shoulders, or kill Sammael." He's clearly only helping Rand because it coincides with some plan of his, not because he particularly cares about Rand's well-being. He certainly doesn't like Rand; when he falls down after the crossed-streams BF incident, Rand offers him a hand, but the Wanderer refuses "with a grimace."
5. [From the RJ aol.com chat, 27 June 1996]: "Question: There is a mystery man who helps Rand in the last chapter of ACOS...is this a new character, or have we seen him elsewhere. RJ: Well, we've certainly seen him earlier in CROWN OF SWORDS."
Given that the Wanderer must be a Forsaken (#2, #3, #4), we are left with only two suspects - Demandred and Moridin. (Osan'gar is not even an outside possibility, since he is Dashiva, and Rand would have instantly recognized him.) The physical description (#1) does not match Demandred (who is not young). In fact, the description matches Moridin quite well. From [ACOS: 25, Mindtrap, 417-418]:
"The speaker was a tall, broad-shouldered young man in black boots and breeches, and a flowing white shirt unlaced at the top, who watched her with startlingly blue eyes ..." He has a deep voice. He has a strong chin, else he'd be worthy of Graendal's collection. He looks to be just a little older than Rand, "Not many years past twenty."
The Wanderer's size, hair color, age, voice, and fashion sense all match with Moridin's, as does his TP addiction. The attention to and interference with Sammael's plans also agree with what we know Moridin (and the Watcher) have been up to. RJ's remark that the Wanderer is somebody we saw earlier in ACOS certainly works if he is Moridin. Furthermore, his remarks to Rand, which indicate that he regards Rand as a tool or a piece in a game, bring to mind Ishamael's comments to Rand in the first three books, and also Moridin's analysis of the sha'rah game in [TPOD: Prologue, Deceptive Appearances, 42-44]. All of this indicates that the Wanderer is Moridin.