Long story short? There was this guy, Stefan Salvatore. When he died, his soul was sent to another ["dimension"] life and started anew.

That life was mine, Stefan Alesci's. So every so often, I remember things about Salvatore. People, places, memories... sometimes even objects, if they're not dangerous.

[He gestures again to the diary] This was one of them. Showed up in my room, right before Atroma grabbed me.