One of the chief of the Valar, and one of the most powerful, ranked only behind Melkor and Manwë. Before the making of the World, he delighted in the making of music, and played a great part in the Ainulindalë, the Music of the Ainur at the beginning of Eä. Within Arda, his special province was the waters of the World, and those waters were said to carry an echo of the Great Music.
Though Ulmo was an ally of the other Valar, and followed Manwë as the Elder King, he did not himself dwell in Valinor. Instead he moved through the deeps of the oceans, though he would come at times to Valmar and sit in council with his peers. Among the Maiar who followed him were Ossë, Uinen and Salmar, and it was Salmar who made Ulmo's fabled horns of shell, the Ulumúri.
Even after the Darkening of Valinor and the Exile of the Noldor, while the other Valar turned away from Middle-earth, Ulmo remained faithful to the Elves and their new allies, the Edain. His power still extended through all the rivers and streams of the World, and so he was able to send messages and inspiration through the music of the water and through dreams. At times, Ulmo would appear himself to those who gained his special favour. On the western coasts of Nevrast, he appeared to Turgon, and sent him on the journey that would lead to the founding of Gondolin. Long afterwards, in the same place, he spoke to Tuor, sending him with a warning to Turgon that his marvellous city would soon fall. He also appeared to Círdan from time to time, sending messages to other Elf-lords of Beleriand.
As the power of Morgoth grew in Middle-earth, Ulmo's own power began to fail, and he could no longer aid Elves and Men as he once had. He had already sown the seed of their salvation, though, by sending his messenger Tuor to Gondolin. There, Tuor had a son, Eärendil, who travelled the Sea to Valinor to sue for pardon and aid. Ulmo joined the other Valar to consider his plea, speaking strongly in Eärendil's favour. It was that council that led to the armies of the Valar marching against Morgoth, and the eventual destruction of Angband in the War of Wrath.