The Battle of River Thracia is an event occuring in the backstory of Fire Emblem: Thracia 776 and elaborated on in the designer's notes of that game.
Following the deaths of Prince Quan and Ethlyn, along with a substantial portion of Leonster's Lance Ritter, at the hands of King Travant of Thracia and his Dragon Knights during Chapter 5 of Fire Emblem: Genealogy of the Holy War, later dubbed the Yied Massacre, Thracia finally had the advantage it needed to begin the subjugation of the Manster District in Northern Thracia.
With Leonster's army largely decimated and the loss of the Gáe Bolg, Thracia quickly seized the chance to launch an invasion, starting with Manster. King Calf of Leonster, alarmed at the new-found aggression of Thracia and saddened by the loss of his son and heir, gathers his vassal Finn and rallied the troops of the other kingdoms of the Manster District, including Alster, Conote and Manster to join him in the defense of their lands. Carl, the young King of Conote, sent his trusted vassal, Raydrik, a powerful noble who supported Carl's enthronement. Calf recognized that Raydrik may not be trustworthy, but welcomed the strength and numbers of his troops.
The forces of the Manster District and Thracia finally clashed near the River Thracia. During the battle, Raydrik, confident in his ability to use the young King Carl as a puppet, ordered his troops to simultaneously take Carl as a hostage back in Conote, and to attack Calf's forces from the rear. Betrayed by his allies, King Calf managed to fight Raydrik himself, but ultimately perished.
After defeating the defending King Calf's remaining forces, Thracia then attacked Leonster Castle, and set it ablaze, and for a short time, Thracia took control of the Manster District. However, Grannvale, now under control of Arvis after his victory at the Battle of Belhalla, invaded Manster (with one of the victims being Grahnye, Eldigan's widow and Ares' mother), and forced Thracia to sign a treaty which annexed it as a territory of Grannvale. He then installed Duke Bloom of Freege as its new King. As a favor to Raydrik for his role in Leonster's downfall, King Bloom made him the Duke of Manster.
During all of this, Quan and Ethlyn's younger son, Prince Leif, managed to evade the new Grannvale Empire's forces that tried to kill him, thanks to the efforts of Quan's loyal pupil and retainer Finn, among others. During the events of Thracia 776, Leif and his allies finally defeated Raydrik, along with his Loptyrian Cult ally, Veld, at Manster Castle.