The rather demonic-looking monster might seem an unlikely face to find on the door of Durham Cathedral. The metal knocker was designed to deter evil from the Cathedral, maintaining it as a place of sanctuary.
In fact, the name of this sanctuary knocker refers to the very specific concept of sanctuary in the middle ages. A person who had committed an offence could seek sanctuary in the church for a relatively short period - weeks rather than months - during which they could choose either trial or exile. In other words, it was a time to get their affairs in order and decide on their future rather than an indefinite escape from the consequences of their actions. Only in the fifteenth century would Durham Cathedral be able to offer permanent sanctuary to debtors and wrongdoers - a privilege limited to certain important churches who held a royal charter. However, it was a sort of imprisonment: the sanctuary-seeker was only immune from arrest while within the cathedral precincts, and had to have the means to support themselves there since they were unlikely to find work.
The knocker now on the door is a replica of the twelfth-century original - still in the cathedral, but now kept in its museum rather than exposed to the elements.