The Yied Massacre is an event which occurs in Chapter 5 of Fire Emblem: Genealogy of the Holy War, referring to Travant's ambush of Quan, Ethlyn, and Leonster's Knight Squadron, the Lance Ritter, during the Gran Calendar Year 761. Travant was hired by Arvis to carry this out, and he killed everyone in the Knight Squadron as well as Quan and Ethlyn - except for their eldest daughter and heiress Altena, who he raised as his own daughter so that she could wield the Gáe Bolg for him.
A more acute description of the Yied Massacre is like this: Travant and his dragon knights first had the Knight Squadron decimated, then Ethlyn (whom Travant seemed to have small feelings for) was killed as well, and then Travant himself grabbed the three-year-old Altena from the side of her mother's lifeless body, using her as a hostage and threatening Quan with killing her as well if he did not surrender the Gáe Bolg. The devastated Quan did so to save Altena, but Travant killed him too, and took flight with his squad, taking Altena and the Gáe Bolg to Thracia.
With Leonster's army largely decimated and the loss of the Gáe Bolg, Thracia seizes the chance to launch an invasion of the Manster District, starting with Manster. After defeating the defending King Calf of Leonster's remaining forces at the Battle of River Thracia, Thracia then attacks Leonster Castle, and sets it ablaze, and for a short time, Thracia takes control of the Manster District. However, Grannvale, now under control of Arvis after his victory at the Battle of Belhalla, invades Manster (with one of the victims being Grahnye, Eldigan's widow and Ares' mother), and forces Thracia to sign a treaty which annexes it as a territory of Grannvale. He then installs Duke Bloom of Freege as its new King. As a favor to Raydrik for his role in Leonster's downfall, King Bloom makes him the Duke of Manster.
During all this, Quan and Ethlyn's younger son Prince Leif manages to evade the new Grannvale Empire's forces that are trying to do him in, thanks to the efforts of Quan's loyal pupil and retainer Finn, among others. About 17 to 18 years later, Leif would be reunited with his sister Altena (now the Princess of Thracia and one of Travant's lieutenants), convincing her to join their cousin Seliph's Liberation Army, then exact his revenge on Travant for the murder of his parents.