Christmas truce

The Christmas Truce was an event that took place around Christmas 1914 during the World War I. During this time, both the German and British soldiers stopped fighting to have a peaceful Christmas.

Erich Albert a German General, took two medics, one German and one British, into his camp during the Christmas Truce, hoping they could help one of his wounded soldiers. The German medic returned to the festivities after the operation, but the British medic, secretly an Assassin, remains behind as Albert offers him his ring. The Assassin noted the Templar seal on the ring and killed the General with his Hidden Blade.

Reference

 * Assassin's Creed: Project Legacy - Holidays: Chapter 1 - Ghosts of Christmas Past