Clara.

I'm going to try now and tell you what really happened in the only way it can - with one of those typically brilliant Steven Moffat twists - what really happened can only be told as a story.

 And so, boys and girls. If you're sitting comfortably.... 

Once Upon A Time  

There was a hansom Timelord who travelled through Time & Space in a Box. He didn't travel alone, he had an intelligent and very pretty companion to keep him company. Clara, with whom he'd shared many, many incredible adventures. There was no such place as Leadworth and neither had ever heard of River Song.

So many incredibly small minded, miserable farts at Gallifrey Base were so happy about living in that world, really. You have no idea.

Then one day the unthinkable happened. Steven Moffat taking over was just coincidence. 

Something hostile over ran the Tardis. Totally. Seized it in a grip it would never let go of and left the Doctor with no choice at all. Non-what-so-ever. He couldn't allow the Tardis to fall into the wrong hands, not knowing what this creature would do with it if not stopped there and then.

Rather than allow that to happen the Timelord did the only thing he could do and blew up the ship.

Clara was trapped in the Tardis Library. 

If a non-belligerent life form is detected in a room about to be deleted the Tardis tries to deposit it in the Main Control Room - but during a self-destruct the Main Control Room is ejected into a time-loop in order to try an preserve the life of the pilot. The Tardis scooped her up easily enough, but had nowhere on board the ship to send her - so it tried sending her off-ship, somewhere safe.

An exploding Tardis exists in all points of time and space. 

She was basically sent everywhere. Endless duplication. 

The original Doctor got to watch her die (or so he believed) on the Control Room monitor -  over and over, forever. 

And that was how things ended. 

Or at least, that's how they would have ended, if what had taken over the Tardis hadn't stepped in. It wasn't about to let the Timelord cheat it out of its prize. Not when it had come so close.

In the Library was a book, the original of this:

In the fraction of a second it took to absorb the content it immediately acquired the knowledge it needed about how to use a Tardis to rewrite time, even if all the time there is is just one single moment - a time machine can use it.

It viewed itself a God. An angry one, cheated. In this new History he wasn't just going to kill the Timelord this time around. That would be a kindness. No. This book was more fitting. This book described how a true God extracts revenge:

And so this was how River Song was originally conceived and came to be - Pandora and all she entailed made real. 

"Trust you to fall in love with a woman from a book" Rory Williams would joke in the reality that would be created as the Doctor sat in the park with him and Amy that last time reading Melody Malone, neither realising how literal and true that statement was or could possibly be. 

River was created as a weapon to extract revenge. 

But what the mind that created her never considered, being the God that it is, that the woman it created to extract revenge for it might also have idea's of extracting revenge for herself - and everyone she loved.

Of course, I could tell you more.

But spoilers, sweety. Spoilers....

If you're ever lost or in doubt - Go back to The Library. The answers are all there.