Isshin Ashina founded the Ashina clan after a bloody Battle twenty years before the events of Sekiro. Old age and disease has caught up to Isshin and he fights to ward off spies and overeager grandchildren.
After meeting Wolf, he gives him the nickname Sekiro, which translates as One Armed Wolf.
Real World Counterpart
The Ashina clan is based off a real clan also founded during the bloody Sengoku period.