What spell killed Nymphadora Tonks?

Nymphadora Tonks was killed by a Killing Curse (Avada Kedavra) cast by Bellatrix Lestrange during the Battle of Hogwarts. This dark spell instantly ended her life, along with that of her husband, Remus Lupin, shortly after. Their deaths highlight the tragic losses faced by the protagonists in the Harry Potter series during the fight against Voldemort and his followers.