Based on Season 4, Episode 3 "In The Beginning"
Prompt: "Destiny can't be changed, Dean – all roads lead to the same destination" – Castiel
Castiel's life span had so far been infinite – while the body he was currently inhabiting had an expiration date, in his true form he would outlive the earth. Quite literally.
He had been around when dinosaurs populated the planet (contrary to popular biblical belief), he had witnessed the first Sin, seen the fall of Troy, attended the premiere of Romeo and Juliet at the Globe theatre, and he had stood in the fields of France during World War II. He had seen everything there was to see, but above all he noticed the patterns.
Two souls throughout history had drawn his attention time and time again – each reincarnation more bloody in their fight to reach one another; each time more deadly in their all-consuming love for one another.
Antony and Cleopatra had been just one reincarnation – and Castiel had just stood there obeying the rules not to interfere. For Destiny could not be Changed.
And from the second he had pulled Dean from the pit, he knew he had found one half of that never ending turmoil. That on-going sacrifice for the lives of each other – and so here he was, returning him to Sam.
Now he just had to wait, for here they were - two souls bound by blood and a love most poisonous. Destiny could not be Changed: it was only a matter of time.