Oromë the Hunter
Arda is the "World" in which JRR Tolkien's legendarium/mythos is set. It includes the places first created by the musical envisioning of the spirit-like ainur, who were inspired by Eru, the One.
In the First Age, Arda included Aman, a vast land where the valar and maiar (both of them groups of ainur who chose to exist bodily within their creation rather than remain in the void) dwelt, as well as the elves who were awoken at Cuiviénen and who, heeding the call of the vala Oromë, came westward across the sea to abide there with them. The island of Tol Eressëa, and the great lands of Beleriand, as well as the Encircling Sea, were also a part of that world. Many long eons passed during this Age, and tales which do not come into this telling are plenty.
Arda was then remade in large part after a war between the valar and Morgoth, a fallen vala who had enslaved or ensnared much of Beleriand, which had become a home for men and dwarves, and other races besides. The new "continent" of Eriador (where the tales of The Hobbit and The Lord of The Rings take place) was wrought from the upheavals of Beleriand, and the new island of Númenor created for men, much as Tol Eressëa was given by the valar over to the elves. Morgoth was cast back into the void, and banished from returning to Arda.
The Fall of Númenor
Trouble again cast a shadow upon the Second Age. Many maia had followed Melkor (the name of Morgoth before his fall) into Beleriand to cast their lot with him. They were the balrogs and a maia called Sauron who grew in favor to become Morgoth's chief lieutenant. With Morgoth gone, most of his followers returned to Aman to seek forgiveness from the valar, and many were granted it, though some were imprisoned within the Halls of Mandos, a sort of limbo. Sauron himself seemed to reform, and was allowed to stay in Eriador, the re-shaped Beleriand. Men grew powerful in those days, and learned much wisdom and many crafts from the elves who had also chosen to live in Eriador. Their lifespans grew many times, and they built huge fleets and massive armies from their cities in Númenor.
Sauron, ever one to seek power, came to be trusted as a source of immeasurable lore by these mighty men, who also grew ambitious, lacking the temperance of elves and growing jealous at their eternal lives and their connection to the undying lands of Aman. Eventually Sauron's councils persuaded the greatest of the sea-kings, Ar-Pharazôn, to mass his armies and navies to sail for Aman itself. This was their great undoing, for they were strictly forbidden to travel west of Tol Eressëa. The valar summoned up a great tidal wave which destroyed their navies and the island of Númenor forever. Only a handful escaped, and those were the wise men among them who had seen the folly of Sauron's councils and had sought refuge east in Middle-Earth (roughly the northwest portion of Eriador) before the catastrophe.
These great men ruled mighty kingdoms in Arnor and Gondor, and at Isengard —where they built the impenetrable tower Orthanc and placed within it one of the palantíri, or seeing stones—and generally lived in peace alongside the elves, dwarves, and 'lesser' men who already inhabited the continent. Yet, Sauron had not participated in the war against the valar but had also returned to Eriador, to stake dominion over it if he could after what he thought would be the sure destruction of the men of Númenor.
When he learned their destruction had not been complete, and that a minority had escaped east (and even a fraction of Ar-Pharazôn's army was formidable) and built these two kingdoms, he took refuge in the wastelands of Mordor and slowly built up a following of his own. He also began to nurture an interest in the craftsmanship of Rings... The Second Age came to its denouement when the mightiest captains of elves and men residing in Middle-Earth besieged Sauron, knowing of his treachery and determined to rid themselves of his shadow, which they had seen growing longer as the years passed. In the Last Alliance of Elves and Men, the great King Elendil shattered his mighty sword against Sauron and was slain, but his son Isildur picked up the pommel and shard and struck Sauron's Ring of Power from his hand.
The tale of The Rings of Power cannot be given space here; but with that loss the power of Sauron was severely weakened and his forces crumbled before the Alliance. And thus ended the Second Age of Arda. The Valar and Eru saw fit to reshape the world again, removing Aman from the world so that no man could ever find it again. Elves alone had that privilege and could still find the way west, but men sailing thither would find no eternal land, only a vast ocean, until at last their ship may come to find the far eastern coast of Eriador. Thus ended the Second Age.
The Gate of Moria
The story of the Third Age was of a long peace finally interrupted by the emergence of Sauron again; first in Mirkwood and later in Mordor, where he sought desperately to find and recover his Ring, which he knew had not been destroyed but only lost. The tales are related in The Hobbit and The Lord of The Rings, which concludes with the destruction of Sauron and his armies, and the crowning of Aragorn II, descended from Elendil himself and therefore of royal Númenorean lineage, as King Elessar of the reunited realms of Arnor and Gondor. The last elves eventually left for Aman to live among the valar for eternity, and the Third Age gave way to the Fourth Age, the Age of Men. Our age.... though long, long ago.
In the next installment I will briefly outline the languages that JRR Tolkien created for his mythology.