Ser Arthur Dayne was a knight of House Dayne who bore the title of "the Sword of the Morning" and a member of the Kingsguard of King Aerys II Targaryen.
Ser Arthur was the second son of Beric Dayne, Lord of Starfall. As the wielder of the sword Dawn, which had been forged from the metal of a fallen star, he bore the title of "the Sword of the Morning".
While skimming The Book of Brothers, King Joffrey Baratheon reads through the entry on Ser Arthur Dayne, mentioning his leading of the attack on the Kingswood Brotherhood and his defeat of the Smiling Knight, one of its members, in single combat. The Book of Brothers also mentions that at one point he was granted temporary command over the Kingsguard after Lord Commander Gerold Hightower suffered injuries. The entry concludes by recounting how he perished at the end of Robert's Rebellion alongside Ser Gerold and Ser Oswell Whent at the Red Mountains of Dorne, fighting Lord Eddard Stark.
Ser Arthur Dayne is seen with Ser Gerold Hightower, the Lord Commander of the Kingsguard under King Aerys II Targaryen, in a vision when Bran Stark sees into the past with the aid of the Three-eyed raven. Near the end of Robert's Rebellion, Ser Arthur and Ser Gerold were stationed to guard the Tower of Joy in Dorne by Prince Rhaegar Targaryen himself. When Lord Eddard Stark of Winterfell arrives with his bannermen, including Lord Howland Reed, in order to rescue the captive Lyanna Stark, Ser Arthur and Ser Gerold Hightower prevent the Northmen from entering the tower, peacefully. Ser Arthur is shown to be cordial to Eddard, but refuses to tell him of his sister, Lyanna Stark. Knowing there is no alternative, Ser Arthur and Ser Gerold fight the Northmen to hold the tower. Gerold is quickly killed by Lord Eddard in the melee, but Ser Arthur easily holds his own against the Northmen, cutting down most of them with two swords, and is able to disarm Eddard. Just before Dayne moves to strike Eddard down, Howland Reed - whom Dayne assumed had been mortally wounded earlier in the fight - sneaks up behind Dayne and stabs him in the back of the neck.
Ser Arthur is mortally wounded by the attack and falls to his knees, holding on to his sword for a few seconds before dropping it as his strength fades. The two look at each other as Eddard picks up the sword, seeming ashamed of how he had survived. Although in agony, Arthur's countenance is neutral, hinting acceptance of his fate and showing no sign of bitterness. With that, Eddard delivers the final blow, killing the legendary Sword of the Morning.
After killing Dayne, Eddard runs up into the tower where he finds Lyanna, dying after giving birth to Rhaegar's last living son, whom Eddard would pass off as his bastard son, Jon Snow.
Considered by somethe finest swordsman in history, ser Arthur Dayne was Kingsguard to the Mad King Aerys II Targaryen. His proficiency with a sword earned Dayne the nickname,"Sword of the Morning". Howland Reed disabled the warrior in battle with a dishonorable move that saved Ned's life and allowed him to finish Dayne off.
Ser Arthur Dayne was a legendary knight and one of the finest men to have joined the Kingsguard, probably rivalled only by Ser Barristan Selmy. He was extremely courageous, leading the attack on the Kingswood Brotherhood and outmatching the Smiling Knight in sword combat - the Smiling Knight had previously defeated a young Jaime Lannister and was considered a terrifying opponent, making Arthur's defeat of him impressive. He is remembered as an excellent swordsman, and when he finally appears when Bran Stark wargs to the confrontation at the Tower of Joy, he is shown to be an incredibly skilled warrior, wielding two swords expertly and cleaving through nearly every opponent he faced; the Three-Eyed Raven pointed out that Arthur was vastly superior to Eddard Stark, and the only reason that he lost was because he was blindsided by Howland Reed.
Ser Arthur's reputation was such that, when people remembered him, it was in acclaim. Years later, Eddard Stark would comment that Ser Arthur Dayne was the best warrior Ned had ever encountered. Even when they outnumbered him, Ned and his companions were extremely wary about how to approach him, showing that Arthur's skill and reputation frightened them. In addition, Arthur was extremely loyal, to a fault, to Prince Rhaegar Targaryen, stating very matter-of-factly that the only reason he and Ser Gerold did not fight on the Trident was because Rhaegar ordered them to remain at the tower. Even after the deaths of Rhaegar and Aerys, he continued to follow Rhaegar's orders to protect the pregnant Lyanna Stark, even if that meant killing her brother Eddard. Although this probably had to do mostly with his vows, it could have also had something to do with the fact that Rhaegar's great-grandmother, Queen Dyanna, was born a Dayne, which could have in turn made Rhaegar and his children, including Jon, kin of some sort to Ser Arthur. Not much else is known about Ser Arthur, but he is shown to be extremely gentlemanly and polite in Bran's vision, greeting his father by title and even wishing him "good fortune in the wars to come"; a hint that despite his skills as a swordsman, even he sometimes, if not always, considered the possibility of losing before fighting someone else to the death. This also suggests that Arthur did not fear death as evidenced by the fact that he didn't beg for his life when Howland Reed wounded him. Instead, he simply faced Ned down before the latter finished him off.