Where does that lore about the Light and the Dark come from?

The Magazine, IIRC.

From WoW The Magazine Issue 4 is:

"It is said that in the beginning there was Light and there was Void and in a time long ago, the two collided in the abyss of the Great Dark.

Infinite worlds spiraled out into the newly forming cosmos... Worlds that would one day bear life forms of wondrous and terrible diversity."

Also, another thread that might be useful: http://www.scrollsoflore.com/forums/...ghlight=Shadow

The magazine is canon?

Yep.

If Light and Void are to be the dominant forces, would the Titans and Old Gods be examples of them? Or are they separate entities? Or perhaps there's some overlap.

Well, the Old Gods were once called parasitic, necrophotic symbiotes. That could mean that they were once creature of the Light, turned into Void.

Dead Naaru? But don't we already have a thing for that?

We are not sure. We know that the Old God are from another plane of existence.

We also know that Naaru and Voidwalkers are tied, because Light can be emptied into Void and viceversa.

The Titans are powerful entities that arose after the beginning of the universe, but Velen's dialogue in the Sunwell epilogue seems to imply that the Naaru (and maybe the Voidwalkers) have existed even before the Material Plane, as they saw the Dawn (the creation).