The answer is: The nearest relative of the victim

Numbers 35:16-21 - The avenger of blood, typically a family member, ensured justice for murder, reflecting God’s concern for justice and order.

In the ancient world, the concept of blood vengeance was as serious as it sounds. If a crime, particularly murder, occurred, it wasn't just law enforcement and justice systems, as we perceive them today, that were involved. Instead, the nearest relative would take on the role of the "avenger of blood." This term reflects much deeper social responsibilities within communities, emphasizing personal accountability and familial loyalty.

In Numbers, as mentioned, God lays down essential laws to maintain order within the tribes of Israel. The idea was that family ties grounded the moral fabric of society. If someone's life was taken, their kin had a vested interest in seeking retribution or ensuring justice. This act wasn't vengeful in the modern sense but was instead a matter of restoring balance and respect to the family of the victim.

Digging deeper, you’ll find traces of this notion in other cultures and their texts. The famous “Code of Hammurabi,” for instance, laid a heavy emphasis on enforcing justice strictly dictated by familial turnover in crime. Also, in many historical Roman laws, there were mandates concerning retribution based on familial connections, highlighting this age-old principle that echo many major religions. Thus, as we observe the framework behind such practices, we find interesting parallels across different ancients civilizations, highlighting humanity’s consistent pursuit of justice and order amidst chaos.