beta
So Hyo un made the black orb that connects their souls forever. The orb retains Hyo Uns memory of his beloved and his experience/love for him. They are somewhat interacting with the flow of time by passing through each memory. Tae fully breaks the barrier between them and the past by stopping Hyo Uns past self from committing suicide. Hyo Uns orb allowed Tae to transcend time and travel back to stop Hyo Un from killing himself. Taehyuk is no longer in just a memory because he directly interfered with Hyo Uns past self. Of course this is just a rationalized theory, lets see what happens