Kind of, it was wayyyy before any of the 40K books. If memory serves, it was the early 90's. It's not in the second ed codex (I have that at hand), so I'm going to have to do some serious trawling through my old stuff that is packed away.
While I adhere to the idea of Ynnead being born powerful enough to put up a fight against (insert random god here) because: A. he´s created/birthed with intent B. the "quality" of Eldar souls is much higher than during the birth of Slaanesh C. there may be bits and pieces of Khaine in the mix I don´t think that would be such a good idea. In fact, I believe the backlash/aftershock of the birth of another god caused by us Eldar would result in the universe disintegrating faster than through the nuke GW dropped on Warhammer Fantasy to create their litte Sigmarines. I guess by killing the universe with the final Eldar death and us being reborn in whatever comes after, we´re still winning but that´s a matter of perspective. Stipulating that Ynnead´s birth is the "final" to the Rhana Dandra, to which Chaos will be present (I´m guessing that includes Slaanesh for soul eating purposes), even if it doesn´t immediately combine materium and immaterium into one, the energy released would probably deal some significant damage to whatever is around at the time in one form or another and punch a hole through "reality". Not saying it would kill any gods but it would hurt and Ynnead might stand a fighting chance. Now, there are still things known and unknown (not written by GW yet) hidden/sleeping in the universe that might react to the ripples such an event could cause, Mag'ladroth being one of them. We can probably agree that a C'tan who´s been taking a ~ 60 million year nap would bring some hurt to pretty much anything in the galaxy (and destoying Mankind simply by getting up from Mars). To make it short, I believe that a "continuous battle" between 1 or more C'tan/any godlike beings with 1 or more Chaos gods would not end well for the universe, i.e. it´s destruction. Not putting a timeframe on any of this, so the whole party could go on for a couple millennia, but total destruction (without anyone winning anything) could be the ultimate result. So with or without reborn Eldar, everything is f*cked ... go Eldar, let´s make a god
Nah, pretty sure the Hive Mind will just clean everything up if we end all up destabilizing the local Material/Immaterial barrier too much (unless we kill it first, please? Like... I think we need a ceasefire to deal with those bugs, their collective hive has a raw psychic ability beyond basically anything that the galaxy's ever seen, most likely). One of the hardest things to accept (at least it was for myself) is despite the power and mythology behind the respective Gods of 40k, they're still just little specks of dust compared to the universe and universally-mirroring Immaterium.
Pfft, bugs are for lesser races. We Eldar will simply conceal ourselves from the Tyranids on our craftworlds and sail onward - let the Orks and Mon'keigh protest the Nids
