Eonwe is the Banner-bearer and Herald of Manwe,
son of Manwe and Varda in early notes, Commander of the Host of
Valinor, and a teacher. He and his sister, Ilmare, are chief among the
might in arms is the greatest in Arda.
Herald and Prophet:
When Earendil the Mariner finally set foot in Valinor and had nearly given up finding anyone, Eonwe appeared on a hill and called to him in a great voice, welcoming him and including prophecies of what he would become. He summoned Earendil to come before the Valar, even Ulmo being called from the deeps, to hear the the request of elves and men for pardon and pity, which they granted. When Mandos asked if Earendil, partly a mortal Man and partly wilfully exiled Noldor, should be allowed to live after setting foot there, Ulmo said he was born into the world for that purpose. Manwe settled Earendil's fate saying Eru had given him the power of doom/destiny, and he decreed that both Earendil and Elwing would live, but not return to the Outer Lands, and that the two and their sons, Elrond and Elros, would be allowed to choose whether to be of the Elves or of Men. Then at the bidding of the Valar, Eonwe set the three companions of Earendil in a boat and sent them home, with a great wind from Manwe to aid their journey home with the news.
Commander of the Host of Valinor:
This granting of pardon and promise to pity meant that the Valar attacked Morgoth without his expecting it, as he still believed the Noldor were estranged from the Lords of the West. Eonwe commanded that mighty host under the white banners in a war called the Great Battle and the War of Wrath. Of elves, this host included the Vanyar elves led by Ingwe along with Noldor elves who had not gone into exile led by Finarfin. The Teleri elves supplied and crewed the ships. The few Men left of the Three Houses of the Elf-friends fought for the Valar, avenging many such as Barahir and Hurin. The Valar and Maiar took on fair and terrible forms, the mountains ringing beneath their feet. They went up, Eonwe's trumpets filling the sky with challenge, against the forces of Morgoth which had become too great for even Anfauglith to contain, so that all the North was at war.
The Balrogs were destroyed, except a few that fled and hid in deep caverns, such as the one Gandalf later encountered in Moria. Uncounted legions of Orcs died, few left for long years after. A great part of the race of Men of Uldor and new from the east marched with the Enemy, that fact never forgotten by the Elves. These men fled east; by using a shadow of fear, they became leaders of the men still there who had refused both the Valar and Morgoth. These men were forsaken by a time by the Valar and were troubled by leftovers of Morgoth's evil things: demons, dragons, misshapen beasts, and Orcs.
Seeing his forces lost, Morgoth in fear loosed winged dragons with thunder, lightning, and a tempest of fire, driving the host of the Valar back. Ancalagon the Black was their mightiest. But Earendil came flying in Vingilot with white flame and about him were gathered all the great birds of heaven with Thorondor as their captain, eagle of Manwe. Earendil slew Ancalagon, the fall breaking the towers of Thangoradrim. Arien, the sun, rose, and the host of the Valar destroyed nearly all of the dragons. The pits of Morgoth were unroofed and broken. Morgoth fled and sued for pardon, but was hurled upon his face and he was bound. The Valar, perhaps Manwe alone, forced Morgoth into the Void, with a guard forever placed at the Walls of the World, and Earendil watching the ramparts of the sky. Yet his influence reaches into Middle-earth until the Last Battle.
Sauron, seeing Thangorodrim broken and Morgoth overthrown, as commander of Morgoth's forces, put on his fair form and went to Eonwe. He did obeisance to Eonwe and repented his evil deeds, knowing what it was to be have once been a Maia of Aule and in fear. Eonwe did not have power to give such a pardon, and commanded that Sauron must go to Manwe for judgement. Sauron, unable to bear what he saw as great humiliation and then possibly receiving a sentence of long servitude, hid himself in Middle-earth after Eonwe's departure. Eventually Sauron fell back into evil ways as the "bonds that Morgoth had laid upon him were very strong".
The two Silmarils still left in Morgoth's crown were taken by Eonwe and guarded.
The battle had been so terrible that the world was changed: the north was rent, the sea roared in through many chasms, rivers perished or found new paths, valleys were upheaved, hills trodden down, and Sirion was gone.
Eonwe as the Elder King's herald called the elves of Beleriand to leave the wreckage of Middle-earth. Many elves agreed, but not all, some lingering many ages such as Cirdan, Celeborn, Galadriel, Gil-galad, and Elrond.
Two chose a different fate entirely: Maedhros and Maglor, the remaining sons of Feanor, sent Eonwe a message that he must give the Silmarils to them, since their father Feanor had made the jewels and Morgoth had stolen them. Eonwe told them that the sons had indeed had that right, but that it had perished due to their many merciless deeds in the name of the oath, especially the murder of Dior and the assault on the Teleri at the Havens. So the light of the Silmarils should return to Valinor from whence it came. He further stated that Maedhros and Maglor should go to Valinor for judgement before the Valar, who alone could decree that Eonwe must give the Silmarils to anyone. Maglor wanted to agree, but was persuaded by Maedhros that if they went to Valinor, they could never fulfill their oath and it had been made to Eru Iluvatar as well as Manwe and Varda who had released them, which would send them to the Everlasting Darkness, the Void.
So the brothers disguised themselves, slew the guards around the Silmarils, and laid hands on the jewels. The whole camp raised against them, but Eonwe stopped them from killing the brothers. Each took a Silmaril and fled. But the Silmaril burned Maedhros's hand unbearably, so that he saw Eonwe was correct, and that the brothers had lost their rights to hold the hallowed stones and the oath was vain. Still carrying the Silmaril, he threw himself into a chasm of fire, dying, and the Silmaril was taken into the Earth.
Maglor, in his own pain, threw his Silmaril into the Sea, and then wandered by its shores ever after singing in pain and regret, his singing only less perfect than Daeron's of Doriath, and never came he back to the other Elves. Thus the Silmarils found their long homes: one in the airs of heaven of Manwe and Varda, one in the fires of the heart of the earth of Aule, and one in the deep waters of Ulmo.
The Men who had always been faithful to the Elves and Valar received Numenor as their reward, beginning the people of the Dunedain from which came Aragorn. Eonwe came and taught them, so that they had more wisdom and power and enduring life than any other mortals have possessed. Numenor lay between Valinor, to which it was closer, and Middle-earth, raised up by Osse from below the Great Sea, established by Aule, enriched by Yavanna, given flowers and fountains by the Elves, with Earendil guiding the Men to the new land with his light. They became taller than any men of Middle-earth, with Elros who was part Man, Elf, and Maia and brother to Elrond, as their first ruler.
References: Silmarillion: "Valaquenta: Of the Maiar", Quenta
Silmarillion: "Of the Voyage of Earendil", Akallabeth: "The Downfall of
Numenor", "Of the Rings of Power and the Third Age"
Art by Turgon-(V) from the Valar Guild's Glittering Caves of Aglarond