What is the origin of the idiom "be after one's blood" and when to use it?

The origin of the idiom "be after one's blood" is rooted in the historical belief that blood was linked to a person's essence, vitality, and life force. This belief goes back to ancient times when people had various superstitions about blood. When someone is said to be after one's blood, it means they are pursuing one aggressively or relentlessly, often with harmful intentions, as if they want to drain one's vitality or cause one harm. It is typically used to describe a scenario where someone is relentlessly seeking to harm, criticize, or get revenge on someone else.