Disclaimer: I don't own Harry Potter, or any characters from the books/ movies.
Starting from the Shrieking Shack:
And then there was an enormous snake, attacking him. Severus let it happen, until Voldemort left.
"Avada Kedavra" he whispered, and the Snake fell down. feeling strangely 'giddy', Severus used his anti snake venom.
"Potter" he mumbled. The Felix Felicis led him to his next actions. He pointed his wand at his temple and waited till the silver substance came out to put it in a bottle. "I am a spy Harry, it was all Dumbledore's idea. "
Harry showed himself, dropping his invisibility cloak. "But you killed him. You killed Dumbledore!"
Even though Harry said that, he looked ready to hear his explanation.
"Harry, he touched a cursed ring, the ring with the resurrection stone. He didn't have long to live. He wanted me to kill him, because he wanted Voldemort to trust me more for his plans to work." He took out his wand, making a silver doe appear.
Harry looked like his eyes would pop out. Severus let out a laugh.
"Harry, I loved your mother, I always will, we were best friends once." He smiled at the memory.
Harry looked even more distressed, probably convinced there was something wrong with him.
Severus handed him the potion. "Dumbledore wanted me to tell you that you must die, at the end." He handed him another potion.
"Felix Felicis potion, or liquid luck, the last time you took it, it benefited you, and not voldemorts part of the soul, so you should be fine."
Harry looked at the bottles. He sighed. "I... thank you...I..."
Before Harry could say something more, Severus again began to speak: "You know, I was prepared to die, to be where your mother is..."
Harry gave Severus a troubled look.
"I'm going to aide you in the back, I have my own ways..." He looked at the invisibility cloak. "To stay... invisible"
"Harry, everything will be fine. Don't forget to take the potion, before facing Voldemort."
That is where it should end, and where Severus Snape will never take a potion that makes him 'giddy' of all things again.
But years after the final battle had ended, Severus used his version of the liquid luck he came up with during the use of felix felicis potion itself, to find someway to time-travel to an alternate/parallel universe, which involved sending his soul over to the body the 'Severus' of his timeline.
Even when using the liquid luck, he knew he had to wait to use the potion again, but he had patience. He also prepared himself as much as possible, even recruiting Hermione Granger of all people. She helped so that 'at least one world had some people living there, who shouldn't have died'.
His preparation involved cramming as much useful information in his head as possible. After years and years of careful preparation, he was finally ready.
He felt his soul leave, and become one with a smaller body then he was used to.