It was the Unseen watcher from Sammys meeting with Sevanna, and we know this was Os'angar. It is possible that Os'angar IS Moridin, which would mean it IS Moridin, but as far as we know Moridin was still in Ebou Dar overseeing Moghdien. It had to be one of the two, if they arent the same, because Rand can detect Sadair being channeled, and can see the weaves of Sadain. We also know it was male, which means it couldn't have been any of the female forsaken, and Lanfear couldn't have been given a new body because she isnt dead. The third possibility is Demandred(who isn't taim), but we have no evidence of his touching the True Power.