World of Darkness
The World of Darkness is a popular game line and game setting published by White Wolf Game Studio. The World of Darkness line includes such games as Vampire: The Requiem (formerly Vampire: The Masquerade) and Mage: The Awakening (formerly Mage: The Ascension).
Relationship of World of Darkness to Ars Magica
Ars Magica is not related to the World of Darkness. IT'S NOT! NOTNOTNOT!!!
Seriously, though, at one time things were heading toward some kind of connection between the two settings. Back when White Wolf briefly owned Ars Magica, they seemed to be working toward the idea that the World of Darkness grew out of Mythic Europe. This appears to be the meta-game reason for such Third Edition features as "True Reason" and the vampires in House Tremere; note also the inclusion of an Order of Hermes in the World of Darkness game Mage: the Ascension (now Mage: the Awakening). It is worth noting that the seminal World of Darkness game, Vampire:the Masquerade, was largely the brainchild of Mark Rein·Hagen, who, with Jonathan Tweet, co-founded Lion Rampant and was part of the original team creating Ars Magica. When Lion Rampant merged with White Wolf, Ars Magica came along for the ride.
That was a long time ago. Atlas Games now owns Ars Magica, and Atlas has nothing to do with the World of Darkness. White Wolf wrote their own medieval-themed World of Darkness "prequel" setting, so there is no need for Mythic Europe to fill that role. The Fourth Edition of Ars Magica eliminated True Reason, and the Fifth Edition removed vampires from House Tremere.
You can still portray Mythic Europe as eventually turning into the World of Darkness if you want to, but there is no longer anything in the official Ars Magica books suggesting that it will (or won't).