Dream B was a drug.
It originated in the Yssgaroth's universe. The drug caused hallucinations and was highly addictive. It was used by the Fellowship during rituals in Victorian London. The Seventh Doctor was surprised to find it being used there, as he believed it was not known on Earth at that time. He had seen worlds in the future devastated by the drug.
Bulbir Singh Mann used a book of William Ashbless' poetry as a hiding place to smuggle Dream B. He brought it to Academician Brown and they shared some of it. After the effects of the drug wore off, he and Mann were killed by General Kopyion. (PROSE: The Pit)