King EärnurEärnur - "When Eärnur received the crown in 2043 the King of Minas Morgul (the Witch-king) challenged him to single combat, taunting him that he had not dared to stand before him in battle in the North.  For that time Mardil the Steward restrained the wrath of the king.  Minas Anor, which had become the chief city of the realm since the days of King Telemnar, and the residence of the kings, was now renamed Minas Tirith, as the city ever on guard against the evil of Morgul.


"Eärnur had held the crown only seven years when the Lord of Morgul repeated his challenge, taunting the king that to the faint heart of his youth he had now added the weakness of age.  Then Mardil could no longer restrain him, and he rode with a small escort of knights to the gate of Minas Morgul.  None of that riding were ever heard of again.  It was believed in Gondor that the faithless enemy had trapped the king, and he had died in torment in Minas Morgul; but since there were no witnesses to his death, Mardil the Good Steward ruled Gondor in his name for many years." ~The Return of the King (Appendix A)


From that point on, the Line of Kings was broken and the Stewards ruled Gondor (until the Return of the King).


"For you must know that much is still preserved of ancient lore among the Rulers of the city that is not spread abroad.  We of my house are not of the line of Elendil, though the blood of Númenor is in us.  For we reckon back our line to Mardil, the good steward, who ruled in the king's stead when he went away to war.  And that was King Eärnur, last of the line of Anárion, and childless, and he came never back.  And the stewards have governed the city since that day, though it was many generations of Men ago." ~The Two Towers, Book IV, chap. 5 (The Window on the West)