You were a mob-hybrid-child, (you choose what kind and age). Dream was walking through a forest, when he found you; an alone, young hybrid child. He thought he could "take care" of you, so he adopted you, and taught you how to survive, and among other things.

Time skip, Dream was put in Pandora's Vault by Awesamdude for all the things he's done; manipulation, torture, and gaslighting and torturing Tommy while he was in exile. You were still young, so you didn't understand anything.