Siege of Mt. Hiei or the Great Fire of Mt. Hiei (比叡山焼き討ち) was a horrible event that happened in 1571 (Genki 2).
After allying each other to fight Oda, the Azai and Asakura used Mt. Hiei's temple as a stronghold ( Battle of Anegawa), but were defeated. Oda, therefore, would not surrender the territory. It was a convenient place near Kyoto.