- "Desperate to cement his stolen throne with an heir, Maegor took three wives at once, known as the Black Brides because each were women he'd widowed in his wars."
"Black Brides" is a term used to refer to the last three wives of King Maegor Targaryen: Elinor Costayne, Jeyne Westerling, and Rhaena Targaryen. They were so-named because they were all widows of men killed by Maegor. The king married them in desperation for an heir, after failing to produce one with his first three wives. All three Black Brides became pregnant, but they all miscarried just like the other three.
In the books
In The World of Ice & Fire and Fire & Blood, Maegor wed the Black Brides in a single ceremony in 47 AC, officiated by the High Septon known as the "High Lickspittle". They were chosen because they were all of proven fertility. Their children were brought to the wedding ceremony as hostages to ensure the brides' good behavior. There are many strange stories about the wedding night, neither of which is certainly true or untrue. Only two of the Black Brides were impregnated - Jeyne and Elinor, and both gave birth to stillborn monsters.
Of the Black Brides (and of all Maegor's six wives), only two survived him - Elinor and Rhaena.
Of the three Black Brides, only Jeyne is mentioned in the A Song of Ice and Fire novels: when Catelyn Stark chides her son for breaching the pact with House Frey by marrying Jeyne Westerling, he says that the Westerlings are better blood than the Freys, and there was another Jeyne Westerling who was queen to King Maegor. Unimpressed, Catelyn sharply points out that will only salt Lord Walder's wounds.