This article contains spoilers for Doctor Who: Time Lord Victorious #1.
Doctor Who's Time War has just been erased from history. When Russell T. Davies relaunched Doctor Who in 2005, he introduced the idea of the Time War - a vast, sprawling conflict between the Time Lords and the Daleks that ended in mutual destruction. It was an inspired idea, because the addition of this new part of Doctor Who lore meant old-school and new fans were on the same page, fascinated by a brand new status quo. Even more intriguing, the Doctor carried the weight of the Time War upon his shoulders, because he was the one who ended it - destroying both Gallifrey and the Daleks.
The truth was eventually revealed in the 50th anniversary special, "The Day of the Doctor," in which viewers learned the Doctor had not destroyed Gallifrey after all. Rather, the Time War had ended with Gallifrey being transported into a sort of pocket dimension, perfectly preserved; the Dalek fleet had been destroyed in the ensuing flare of energy. It was a tremendous twist, bringing the shadow of Gallifrey back into play once again and redeeming the Doctor, meaning he was no longer the one who destroyed his own people. Now, though, it seems all that lore has been erased - because the Time War has been wiped from history.
This week's Doctor Who: Time Lord Victorious #1 is the beginning of a massive transmedia event, kicking off with David Tennant's Tenth Doctor recruited by the Daleks as their last defense. The unlikely scenario is made all the more disturbing when the Doctor realizes history has changed dramatically; while the Daleks are aware of Gallifrey's existence, they have absolutely no recollection of the Time War. Needless to say, the discovery leaves the Doctor shaken.
The Doctor is a time-sensitive, and he could already sense he was dealing with a paradox of some kind. But the scale of this change in time is beyond belief, not least because the history of both the Daleks and the Time Lords was sealed behind a time lock, just as you might cauterize a wound to stop it bleeding. The Time War came close to breaking the entire time-space continuum, and meddling with it - let alone wiping it from the timeline - is to risk all creation.
It's reasonable to assume the change is even bigger, though. The Doctor learns the Daleks are facing a threat from the Dark Times, an era when the Time Lords were far more interventionist and wiped out races like the Racnoss and the Great Vampires. And this is the beginning of the "Time Lord Victorious" arc, which promises to send the Doctor back to those Dark Times - meaning it's entirely possible he is the one who has created this new timeline. That would certainly be the biggest paradox in the history of Doctor Who.