The Black Temple is a location inside Darkling Woods in the game Fire Emblem: The Sacred Stones. It is the location for the final chapter and occurs in two battles. The first battle consists of monsters who are led by Lyon. If you defeat him, he then dies and the Demon King, Fomortiis is resurrected. Before he dies however, in Eirika's route Lyon will confess to Eirika that he loved her and that he willingly let the Demon King possess him in the hopes of preventing the disaster that he foresaw. In Ephraim's route Lyon will tell him that he had envied Ephraim because of his skill with a lance. Ephraim in return tells him he envied Lyon's skill with magic. Then whoever was left from the previous battle fights the Demon King and destroys him forever when they succeed.
The Black Temple is where Grado and his men, alongside Morva, first defeated the Demon King by imprisoning him inside the sacred stones, however they were unable to defeat his body, and it was left to sleep in the Black Temple, guarded by the Manakete Morva.
In the final level of The Sacred Stones, the body of the Demon King is destroyed, and Ephraim notes that they, with the help of the sacred relics, had done more than even Grado had been able to do. They had destroyed the body of the Demon King as well as reimprisoned him in the one remaining stone.