Monkey Apprentices must embrace fire wholly in order to learn the most powerful technique of summoning a fire-breathing Phoenix. The Fiery Pit facilitates this embrace.
~ Official BMC description

The Fiery Pit is a Special Building that is required for research of the Summon Phoenix upgrade. The building can only be placed on a Volcano Terrain tile, and nowhere else. It costs City cash17,000, takes 36 hours to build, and gives 500 XP once construction is complete. It consumes Lightning thing-040 and a 1x1 tile space. It requires level 18 to be unlocked.