The Dream Lord seems to know all about the Doctor. He seems to be an intangible being, with no physical form, a creator of deadly dreams and delusions, who delights in being offensive and annoying.

He seems to be able to take the visions he creates from the imaginations of his victims. He seemed to have taken on the Doctor because he was a great challenge, and trapped him and his companions between two realities, daring them to guess which was real and which the dream. And he seemed to have cheated.

But in reality, the Dream Lord was everything dark in the Doctor, given a voice when a speck of psychic pollen from the candle meadows of Karass Don Slava fell into the console and heated up. And with all his centuries of life, the Doctor's got a lot of darkness to deal with.