Who saved the knight and under what condition was he let go?

The knight was saved by a courageous young maiden who intervened just as he was about to be executed. She pleaded for his life, promising to find a way to repay the king for sparing him. The king agreed to let the knight go under the condition that he would undertake a quest to seek a rare artifact that could benefit the kingdom. Thus, the knight was granted his freedom but now faced the challenge of fulfilling the king's demand.