From The Heroes of Might and Magic III wiki

[edit] Fire Lake

The Fire Lake is the level 6 dwelling of the Inferno town. It produces Efreeti at a rate of 2 a week.

Requires: Mage Guild Level 1, Fort, Imp Crucible, Hall of Sins, Demon Gate

Cost: 4000 Gold, 10 Ore, 3 Gems, 3 Mercury, 3 Sulfur

[edit] Upgraded Fire Lake

The upgraded Fire Lake allows the production of Efreet Sultans

Requires: Mage Guild Level 1, Fort, Imp Crucible, Hall of Sins, Demon Gate and Fire Lake

Cost: 3000 Gold, 5 Ore, 5 Gems, 5 Mercury, 5 Sulfur