Deaths on September 21

William of Kilkenny, Lord Chancellor of England

William of Kilkenny (died 21 September 1256) was a Lord Chancellor of England and Bishop of Ely.

4 May

Pope Alexander IV
The Augustinian monastic order is constituted at the Lecceto Monastery when Pope Alexander IV issues a papal bull Licet ecclesiae catholicae.
15 Dec

Alamut Castle
Mongol forces under the command of Hulagu Khan enter and destroy the Hashshashin stronghold at Alamut Castle (in present-day Iran) as part of their offensive on Islamic southwest Asia.