Ulmo

The Vala of the seas, similar to Poseidon in Greek or Neptune in Roman mythology.

Now to water had that Ainu whom the Elves call Ulmo turned his thoughts, and of all most deeply was he instructed by Ilúvatar in music. The Silmarillion: Ainulindalë

Ulmo was in might second only to Manwë.

Ulmo did not dwell in Aman, but dwells as he will in the deep waters of the world, needing no permenent home.

Ulmo's instrument was the horn Ulumúri

When the other Valar retreated to Aman, Ulmo never forgot Arda, and his force extends into all rivers and streams.

Ulmo counseled the Valar to leave the Awakened Elves to wander free in world and not to bring them to Aman.

Ulmo came to Finrod and Turgon in their dreams and instructed them to build kingdoms 'of hidden strength ' against the coming days of evil.

It was Ulmo the did show to Turgon the secret way to the vale of Tumladen that would become known as Gondolin.

The Ringwraiths have trouble crossing running water. They only attempt it when they know their own powers exceed Ulmo's in a given area.

Ulmo is described thus in Unfinished Tales: Of Tuor and His Coming to Gondolin:

A tall crown he wore like silver, from which his long hair fell down as foam glimmering in the dusk; and as he cast back the grey mantle that hung about him like a mist, behold! he was clad in a gleaming coat, close-fitted as the mail of a mighty fish, and in a kirtle of deep grean that flashed and flickered with sea-fire as hie strode slowly towards the land.

tbc Dtc