What element is the brimstone in the expression fire and brimstone?

Brimstone: Brimstone is yellow, elemental sulfur, which burns with a very very hot blue flame that you can only just see, to give the highly unpleasantly pungent gas, sulfur dioxide. Brimstone is...

Sulphur, the nonmetal periodic element number 16

Sulfur was historically called brimstone. (The "brim" part is from an old word meaning "burn", since elemental sulfur can occur as a rock-like substance that does, in fact, burn.)

Sulfur does. It appears in the form of a compound with oxygen: sulfur dioxide.

Brimstone is the same thing as Sulfur