Battle of River Thracia

The Battle of River Thracia is an event occuring in the backstory of Fire Emblem: Thracia 776 and elaborated on in the game's designers notes.

Following the deaths of Prince Cuan and Ethlin, along with a substantial portion of Leonster's Lance Ritter, at the hands of King Trabant of Thracia and his Dragon Knights during Chapter 5 of Fire Emblem: Seisen no Keifu, 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 Gae Bolg, Thracia quickly seizes 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 rallies the troops of the other kingdoms of the Manster District, including Alster, Manster and Conote to join him in the defense of their lands. Carl, the young King of Conote sends his trusted vassal, Leidrick, a powerful noble who supported Carl's enthronement. Calf recognizes that Leidrick may not be trustworthy, but welcomes the strength and numbers of his troops.

The forces of the Manster District and Thracia finally clash near the River Thracia. During the battle, Leidrick, confident in his ability to use the young King Carl as a puppet, orders 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 manages to fight Leidrick himself, but perishes.

Aftermath
After defeating the defending King Calf's remaining forces, 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 Alvis after his victory at the Battle of Barhara, invades Manster (with one of the victims being Grahnye, Eltshan's widow and Aless's mother), and forces Thracia to sign a treaty which annexes it as a territory of Grannvale. He then installs Duke Bloom of Freege as it's new King. As a favor to Leidrick for his role in Leonster's downfall, King Bloom makes him the Duke of Manster.

During all this, Cuan and Ethlin'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 Cuan's loyal pupil and retainer Finn, among others. During the events of Thracia 776, Leif and his allies finally defeat Leidrick, along with his Lopto Sect ally, Veld, at Manster Castle.