What is a blood letter in the old ceturies?

In the old centuries, a blood letter referred to a type of letter or document in which the writer would sign their name in blood as a symbol of commitment, sincerity, or solemnity. This practice was often associated with pacts, oaths, or covenants, suggesting a deep personal investment in the content of the letter. It was believed that using blood, a vital and life-giving substance, added weight to the promises or sentiments expressed in the correspondence. Such letters were often seen in contexts of loyalty, love, or even magical practices.