Reincarnation Pill: The Retired Assassin

Before the nuclear war of the 21st century, a vampire named Voltaire took a pill for him to escape the troubles of the dystopian reality of our apparent world.

Then, the pill made him dream about his life-long reincarnation to a post-apocalyptic world, where time became medieval again, in a fantasy-like world. Then, he meets his past-self from the current era, as they converse and travel together as one soul but different people, bounded by fate.