- "The Lord let you come back for a reason. Stannis was not the Prince Who Was Promised, but someone has to be."
- ―Melisandre to Jon Snow
"The Prince That Was Promised" (sometimes called "the Prince Who Was Promised" or "the One Who Was Promised", and also referred to as "the Lord's Chosen, "the Son of Fire", and "the Warrior of Light") is a prophesied savior in the religion of the Lord of Light. According to the prophecy, this figure would be born "amidst salt and smoke" and pull a sword named Lightbringer from flames, which they would use to combat an impending darkness. The prophecy was originally written in High Valyrian, and this led to a misunderstanding in translation, as the Valyrian word for to "prince" is gender-neutral. The accurate name for this prophesied savior is "the Prince or Princess That Was Promised", indicating that it could be a man or a woman.
According to Melisandre, a red priestess of the Lord of Light, Stannis Baratheon, Lord of Dragonstone and self-proclaimed King of the Andals and the First Men, is the prophesied Prince. However, after Stannis is killed in battle and Jon Snow, the Lord Commander of the Night's Watch, is raised from the dead, Melisandre claims Stannis was not the Prince, but "someone must be," suggesting she may now believe it is Jon. She later confirms this belief to Davos.
Another Red Priestess, established in Volantis, preaches to a group of slaves and freedmen that Daenerys is their prophesied savior. Kinvara, the High Priestess of the Red Temple of Volantis, is also certain that Daenerys is the one who was promised, citing her freeing of slaves and birthing of dragons from stone as evidence. She agrees to send her best priests and preachers throughout Essos, spreading the good word about the Queen.
After Melisandre is banished from the North, she learns of Daenerys and her dragons. She comes to Dragonstone to make Daenerys aware of her destiny, and to tell her of Jon's importance as well in the coming war.
In the books
In the A Song of Ice and Fire novels, the Prince That Was Promised is implied to be the reincarnation of a hero known as Azor Ahai, a legendary warrior who supposedly lived thousands of years ago. Stories about the Long Night - or a similar cataclysmic event - are told throughout Essos, one version of which says that a great darkness descended upon the land. Wielding a flaming sword called Lightbringer, Azor Ahai rose up and defeated this darkness. There is some debate among fans as to the interchangeability of "Azor Ahai" and "The Prince That Was Promised." It is implied that Azor Ahai is the name given to the legendary figure, while the Prince That Was Promised is more properly his reincarnation. Melisandre has mentioned the Prince that was Promised and Azor Ahai interchangeably, but tends to use the name Azor Ahai far more often. In any case, the books do not make an explicit connection between the stories of Azor Ahai and the prophecies of the Prince That Was Promised.
Moreover, it is never clearly revealed where the prophecy about the Prince originated, or what culture/religion made the original prediction. Legends of Azor Ahai are widespread across the further east of Essos, though he is known by different names, depending on the region. According to Melisandre, who hails from the city of Asshai, the prophecy about the Prince was known there five thousand years ago, but even this was millennia after Azor Ahai supposedly lived.
The phrase "The Prince That Was Promised" is first mentioned in the second novel A Clash of Kings: at the House of the Undying, Daenerys sees a man and a woman - presumed to be her brother Rhaegar and his wife Elia Martell - speaking about a newborn baby that the woman is nursing. The man says that the baby shall be called "Aegon", and that he is "the prince that was promised".
Dragons often figure in various versions of this prophecy, and one interpretation indicates that the prince would be born from the line of House Targaryen. Stannis does match this description, because his grandmother was a Targaryen, but he was not born at a place of "smoke and salt" (which can be Dragonstone, a volcanic island) - he was born in Storm's End. In Daenerys's vision, the man also mentions that "the dragon has three heads", a line repeated throughout the series. This appears to reference the three-headed dragon of House Targaryen's sigil, but has also been interpreted as meaning that the "Prince" is actually three individuals working together.
In the fifth novel A Dance with Dragons, Barristan Selmy tells Daenerys that a woods witch, who was a friend of Jenny of Oldstones, told King Jaehaerys II Targaryen the prince that was promised would be born of the line of his children, and that prophecy made him wed his children to each other.
Although Daenerys's gender should raise doubts as to the Red Priestess's identification of her as the Prince That Was Promised, Maester Aemon points out that in High Valyrian (the language most versions of the prophecy were written in), the word corresponding to "Prince" is gender neutral. Aemon quickly realizes this after Melisandre begins proselytizing upon her arrival at the Wall. Although he didn't realize it during his years of correspondence with Rhaegar Targaryen, Melisandre's identification of Stannis as the Prince makes him reconsider the evidence, if only to shut her up. In the series, Kinvara seems to have come to this same conclusion, while it is Missandei who points out the translation issue on-screen.
In the books, none of the prophesies detailing the Prince That Was Promised match the life of Arya, who kills the Night King, and thus all White Walkers, in the show. However, she fits somewhat with Rhaegar's insistence that "the dragon must have three heads," although as a Stark, the sigil of her house is a direwolf, not a dragon. See "The Long Night" for more details.