The Last Great Time War between the Time Lords and the Daleks, the Eternals were so affected by the violence and the damage to space and time that they left the mainstream five-dimensional universe completely, never to be seen again.
Except one escaped, Death. Now calling herself Missy. Not before she stole the Time Lord Matrix, uploading the minds of the people the Doctor could not save, not just ordinary people but people that were soldiers. The Doctor hated soldiers, what better to use against him.
Ever since she met him as a little Galifreyian boy, coming to him after he killed another boy in self defense, she knew what he would become. Death would follow him everywhere. She wanted him as her own someday.
The boy had been bullying young Theta Sigma and Koschei. Both of the boys, got rid of the body by pushing it off a cliff, to look like he had fallen.
She had came to the Doctor afterwards in a dream and offered him a chance to be free of the guilt and memory of this crime, but that if he asked for it his best friend would be stricken with this guilt and memory instead. According to Death, the young boy who would become the Doctor agreed to this bargain and his friend believed he had done the killing instead. The friend would later be known as the Master.
When his time came, on the fields of Trenzalore, you could imagine her disappointment, her anger, the he was able to cheat death, escape her once again.
To keep track of him, she made sure Clara stood by him. When the time came to finally meet him, she would finally take his soul. If he didn't come to her willingly, she would kill every human on this God-forsaken rock if she had to.
AN: Those not familiar with Classic Doctor who, the Eternals are a race of cosmic beings first introduced in the Doctor Who episode Enlightenment, back in the 5th Doctor era. The Eternals lived outside of time, in the realm of eternity.