But why, exactly, has Gondor not had a king for so long and why is Boromir, as the son of the Steward, seemingly angered by the reveal of Aragorn’s true lineage? The story of the line of Elendil seemingly breaking is a tragic but fascinating one, going all the way back to Witch-king’s challenge unto Earnur, Prince of Gondor.
Most people consider Sauron to the “main” villain in Middle-earth and while that is certainly true for Tolkien’s most popular works, The Hobbit & The Lord of the Rings, there were actually many other villains that rose and wreaked havoc across Middle-earth during the ages were Sauron was working to regain form.
In particular, the Witch-king of Angmar rose to prominence and was the foremost antagonist in Middle-earth for the bulk of the Third Age until the War of the Ring. Ruling from his keep, Carn Dum, in Angmar, the Witch-king and his forces continually battled against the split realms of Arnor; Arthedain, Cardolan & Rhudaur (there will eventually be another post on this history,) but for over a thousand years Angmar sought to conquer the Northern Kingdom. One of the most major battles occurred in TA 1974, when prince Earnur of Gondor sought to reclaim Fornost, what once was the capital of Arthedain after Annuminas was abandoned, from Angmar. (Boromir is actually compared to Earnur in the books, who also excelled at strength of arms and held no love for lore, save for tales of battle and victory).
Earnur led his own Gondorian soldiers as well as the remants of the Dunedain, elves led by none other than Glorfindel and according to the scholars of the Shire, some halflings joined the battle as archers. The armies of the Free Peoples and Angmar met on the shore of Lake Evendim and during the battle, the Witch-king rode forth and revealed himself to Earnur. He challenged the prince and as Earnur rode to meet him, his horse was frightened and bucked him to the ground. The Witch-king taunted him but fled all the same, for his forces were overwhelmed, and Earnur charged after him only to be stopped by a prophetic warning from Glorfindel. That prophecy, which stated the Witch-King could be killed by no man, came true during the Battle of the Pelennor Fields, when Eowyn slew him in defense of her uncle.
Though the battle was won by the Free Peoples, nonetheless the Northern Kingdom had been all but destroyed in the Witch-king’s wake. The Witch-king then fled to Mordor and from there he grew in power once more, eventually seizing Minas Ithil, sister city to Minas Tirith, and corrupted it, renaming it Minas Morgul. In TA 2043, 51 years after the Battle of Fornost, Earnur’s father died and he inherited the throne of Gondor. The Witch-king once more challenged Earnur, taunting him, and the King was only held back by the counsel of his Steward, the wise Mardil Voronwe.
However, seven years later, the Witch-king challenged Earnur once more and he accepted, ignoring the advice and pleas of his Steward. Accompanied by a small company, he passed through the gates of Minas Morgul and was never seen again, but was believed to have died in utter torment at the hands of the Nazgul. Earnur had no heirs and to save Gondor from falling into civil war, Mardil Voronwe stated that he and all in his line would rule “until the King returns”.
In short, Gondor has neither a king nor even hope of a king for 969 years, until Aragorn is crowned after the War of the Ring. Because of this, Boromir is first and foremost startled to hear that there is indeed an heir to the Throne at the Council and also, to put it lightly, frustrated by his sudden appearance from near an age without a king.