The Valaróma was the name of Oromë's great horn, the sound of which was like the rising of the Sun in scarlet, or lightning cleaving the clouds.

The Valaróma was made by Salmar , and it could be heard above all other horns. When the Eldar heard it, they knew that Oromë was once again pursuing the servants of Morgoth.
Encyclopedia entry originally written by atalante_star