A golden powdery substance
Dream dust is a golden powdery substance that, when sprinkled in small amounts, causes people to dream. Sandmen use it to give all human dreams, both magical and mortal, to work out inner conflict. Neither dream dust nor Sandmen can control the dream and what happens in it.
When too much is used on a person, figments of their dream can come out of their subconscious minds and into reality. Sandmen are made of entirely of dream dust, so if they are vanquished in any way, their bodies explode into a pile of dream dust.