Draakhorn

The Draakhorn was once the horn of Tiamat’s ancient red dragon consort, Ephelovax, which she gave to the dragons to help them in their war against the giants. The Draakhorn is a signaling device, and it is so large that it requires two Medium creatures (or one Large or bigger creature) to hold it while a third creature sounds it, making the earth resonate to its call.
The low, moaning drone of the Draakhorn discomfits normal animals within a few miles, and it alerts all dragons within two thousand miles to rise and be wary, for great danger is at hand. Coded blasts were once used to signal specific messages.
Those with knowledge of the Draakhorn’s history know that it was first built to signal danger to chromatic dragons—a purpose the Cult of the Dragon has corrupted to call chromatic dragons to the Well of Dragons from across the Genabacis.