Sulfur

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

In Dungeon towns, you can build a Resource Silo which will bring you 1 unit of sulfur per day.

Calid the Heretic is able to produce 1 additional unit of sulfur per day for the Kingdom he serves. Also, any hero equipped with the Eversmoking Ring of Sulfur produces 1 unit of sulfur per day (Calid would produce 2).