According to the Glossary in Lord of Chaos:
"Once a peddler trading into the Two Rivers, and a Darkfriend, he was transformed at Shayol Ghul not only to enable him to find the young man who would become the Dragon Reborn as a hound finds prey for the hunter, but to ingrain the need to find him. The pain of this induced in Fain a hatred both of the Dark One and of Rand al'Thor.
Fain encountered the trapped soul of Mordeth in Shadar Logoth as he was on the trail of Rand al'Thor. Mordeth's soul attempted to seize control of Fain's body, but the result was a combination of the two evil souls. Fain remains mostly in control, but he now has abilities and strengths that exceed anything that either man had before.
For a time, Padan Fain (using the name Ordeith) "served" the Lord Captain of the Children of the Light while he worked his campaign of cleansing the Two Rivers of Darkfriends. Fain, on the other hand, brought an army of Trollocs and Myrddraal in to the region. He later disappeared, only to resurface as Jeraal Mordeth, an advisor to Lord Toram Riatin in his rebellion against Rand al'Thor (the Dragon Reborn) in Cairhien. Again, Fain brought death and destruction to the area. He attempted to kill al'Thor, stabbing him with the cursed dagger from Shadar Logoth, but disappeared into the killing mist he had brought upon the Rebels. His present whereabouts are unknown.