Sulfur

Sulfur is one of the four magical resources in Heroes (along with Crystal, Gems and Mercury). Sulfur are needed for several buildings, recruiting Red Dragons, Black Dragons, Chaos Hydras and Rust Dragons. The primary source of Sulfur is the Sulfur Dune, which brings its owner 1 unit of Sulfur per day. You can also find Sulfur scattered around the land in small amounts (3-6 units of Sulfur per resource stack).

In Dungeon town you can also build the Resource Silo which will bring you 1 Sulfur per day.

Calid is able to produce 1 additional Sulfur per day for the Kingdom he serves. Also, any hero equipped with the Eversmoking Ring of Sulfur gives you 1 Sulfur every day.