From The Heroes of Might and Magic III wiki

The Cover of Darkness creates a permanent shroud of darkness over your town. The shroud is only visible by opposing players and regenerates each day. The effect is similar to the Cover of Darkness on Adventure Map (A sphinx on a platform emitted the shroud.) A Tower town with a Lookout Tower will be able to use it to automatically penetrate the darkness, whereas a Skyship can't be used for this purpose.

Requires: Fort

Cost: 1000 Gold