"The brimstone, too, which burns there in such prodigious quantity fills all hell with its intolerable stench;"
Burning sulfur
Creative Commons Attribution Share AlikeBurning sulfur - Credit: Johannes Hemmerlein

Brimstone is an alternative name for sulphur, a chemical element associated with volcanoes. This has given rise to the term 'fire-and-brimstone' sermons, in which listeners are reminded of the hell that awaits them if they do not repent their sins.

Pure sulphur does not smell. The 'rotten egg' stench comes from hydrogen sulphide, which is generated  by bacterial decay in anaerobic conditions such as swamps.

When burned, sulphur produces a red liquid and a blue flame.