Short answers, pulled from the story.
The cornu was an ancient Roman brass instrument about 3 meters long, curved into the shape of the letter G. It was used by the Roman army as a signaling instrument to communicate orders to troops in battle, and also appeared at gladiatorial games, religious rituals, funerals, and sacrifices.
The cornu was invented by the Etruscans, who used it in their funeral processions and military. It was likely a status symbol in Etruscan society before the Romans adopted it for military, religious, and ceremonial purposes.
The cornu was originally made from an animal horn and later constructed from bronze. Metal sheets approximately 0.5 millimeters thick were shaped into a spiral, joined by soldering, and connected by brass rings; the bell at the end was made from copper and tin.
A cornicen was the Roman musician who played the cornu. The player held the instrument vertically with the tubing passing around the left shoulder, gripping it with the left hand while pressing the mouthpiece to the lips with the right hand; pitch and volume were controlled entirely by breath and physical force.
Two specimens of the cornu survive in the archaeological record, both recovered from the ruins of Pompeii.
The cornu was revived during the French Revolution under the name tuba curva. It was first used alongside the revived buccina in music composed by Francois Joseph Gossec for the translation of the remains of Voltaire to the Pantheon on the 11th of July 1791.