- "Legend has it that the Dornish founder of House Dayne followed a falling star to where it hit the ground. And there, he raised his castle. A fantastic story, I know, but House Dayne has a relic to prove it. Dawn. The most famous sword in Westeros."
- ―Eddard Stark
In a vision witnessed by Bran Stark and the Three-Eyed Raven, Ser Arthur Dayne wields Dawn alongside a second sword in his fight against Eddard Stark and his bannermen during their encounter, slaying most of them and wounding Howland Reed. He duels with and disarms Ned, but before he can kill him, Howland stabs him through the back of his neck. Ned then takes up Dawn and slays the mortally wounded knight with his own blade.
When Bran continues his vision of the Tower of Joy some weeks later, he sees Ned holding Dawn, still covered in Ser Arthur's blood, as he climbs the stairs of the Tower of Joy. Ned then rests the sword at the foot of the bed in which Lyanna Stark is lying.
In the books
After the showdown at the Tower of Joy, Eddard returned the blade to House Dayne at Starfall; he presumably does this in the TV series as well, as he makes sure to take the sword with him when he leaves the battle.
According to George R.R. Martin, Dawn remains at Starfall castle during the War of the Five Kings, because no new warrior of House Dayne has yet proven himself worthy to wield it as a new Sword of the Morning. Martin stated that if he could wield a sword, he would choose Dawn, because it was made from a falling star and "who knows what magical properties a falling star has."
In the novels, Dawn is a Greatsword similar to Ice, which Ser Arthur wields with both hands, whereas in the show, the Greatsword (which seems to be scaled down in order for the Actor to work with two swords at once) is wielded alongside a second blade in the off-hand in order to display Arthur's skill at swordsmanship. There is no mention in the novels that Ser Arthur ever used Dawn in this fashion. With no desire to narrate a lengthy exposition on Dawn and the Sword of the Morning, this change in combat style was likely made to enhance the abilities of Ser Arthur for the show, since dual wielding is a popular and powerful technique in fantasy RPGs. Dual wielding is actually quite difficult in real swordplay, unless the blades are both relatively short, or one blade is long and the other very short (in which case it is usually a specially designed parrying dagger). Again, this is presumably done in order to draw attention to Ser Arthur's prowess.