Dartain is how. and it was after Odus was thrown to Ultera. Dartain figured out a way to steal souls/power/knowledge and absorb it into himself to make himself more powerful. he tested it on things like kobolds and other erudites...then tried to take the essence of Al'Kabor from his resting place. Al'Kabor turned out to be, even in death, a VERY powerful mage, and was able to resist Dartain's ability/attempt to absorb him...fighting back and forcing Dartain to come to a sort of truce. now they share the body, and it shifts back and forth between their forms and they are the Duality.
@Caedo Thanks a ton for asking the question. I'd wondered that myself. @Rainmare is that some of the lore that is/was available only to raiders? I know for a couple of expansions/updates only the raiders have been privy to the storyline. Does anyone know if there are lore books we casual folk could purchase that would help up with the storyline(s)? I will eventually get my main to 95 but am really interested in the lore on the way.
you learn about it in the Odus sig line that starts in Paineel, the 'emergence of Dartain' line. it was entirely group/solo stuff, like the Ethernaught line.
something that bugs me thogh... is how did Dartain have time to head to Neriak for the necromancer epic/mythical quest line if was working in Ultera the whole time.
my guess would be that Ultera and Norrath time doesn't flow the same way. it coudl very well be that Dartain being in Neriak was while he was studying things to attempt to merging with Al'Kabor. remember the Feedback/Luclin going boom was like 45 years before eq2...but there are erudites in Paineel that claim to have been there 100s of years. (well past thier normal lifespan I wager) So Dartain spends a few decades in Ulterea, heads to neriak (which for use was only a few years at best) learns what he can from Neriak's library, heads back to Ultera, attempts his merge on Al'Kabor...becomes the Duality with him.
it was designed that way, so that everyone can do it. rather then gating people behind group/raid content that some people never get to access. ie Fallen Swords, where barely a handful of people on the server have ever seen durnder, mmuch less done all 4 of the drunder instances.