The Battle of the Catalaunian Plains was a major battle between the Roman army and the Huns. It took place in AD 451 in Gaul between Orléans and Chalons near Catalaunum, which was incorporated into the Hun camp.
It was a fixed point in time.
Both sides were manipulated into the battle by the Tenctrama so as to achieve maximal casualties. In particular, they made sure that the slain soldiers were rising from the dead under full control of the Tenctrama. The dead from each side did not attack each other but tried to kill more living. The Tenctrama also created fearsome undead animals called the Strava by DNA manipulation. These beast also attacked only the leaving.
The Roman army was under the command of Flavius Aetius whereas their allies Visigoths were initially led by their king Theodoric. When he was killed by a Tenctrama, his son Thorismund took the charge.
The Thirteenth Doctor and Team TARDIS visited the battle shortly before its beginning. Soon separated, the Doctor and Yasmin Khan were taken prisoner by Attila, who demanded from the Doctor to provide him with more powerful magicks in exchange for keeping Yaz alive. Graham O'Brien was brought to Flavius Aetius and was ordered to cure badly wounded Theodoric. Ryan Sinclair became friends with Licinia Postuma, a member of the Legion of Smoke.
When both fighting sides realised that the dead kill all living, including their former brothers-in-arms, Attila, Flavius and Thorismund were forced to fight against the dead together and were persuaded by the Doctor to agree to a truce and order their respective armies to retreat to prevent strengthening the Tenctrama by further deaths. In the end, the Doctor was able to stop and destroy the Tenctrama.
History books did not preserve the details of the battle but were aware of its unprecedented scale: historians believed that three hundred thousand soldiers died in a single day. The results of the battle were considered indecisive. The Tenctrama influence was reflected in the legends claiming that ghosts of those who fell continued fighting for three more days.