Of the world of Eberron, the great nation of Cyre was the former home of House Cannith.
The area it existed in is now more commonly referred to as the Mournland, due to the apocalypse, called the Day of Mourning, that was visited upon it at the end of the Last War.
Today that land is covered in a grey mist cloaking deserted cities and a curse that prevents both most healing of the living and all decay of the dead. It's a place where creature tend not to survive, and those that do survive are changed physically, mentally and spiritually.
Sometimes "the Mournland" is called "the Mournlands" in-game.
See also Mournlands.