The cloud is comprised of brimstone, acid, and poisonous gases. That is what it literally says. Those are the components. It also says it is a mix of the stench of rotting corpses and the essence of evil. The essence of evil can be considered the magic used to summon the components, may also be in the Hell brimstone. The stench of rotting corpses comes from the chemical byproducts (Putrescine and Cadaverine) released as microorganisms consume dead flesh. These chemicals are poisonous, causing decay but not in a magical necromantic sense (which wasn't added to the lore until much later).