Final answer:
Regulus Black, Sirius Black's brother, died while trying to retrieve Voldemort's Horcrux guarded by Inferi as part of his stand against the Dark Lord.
Step-by-step explanation:
Regulus Arcturus Black, the brother of Sirius Black, died while attempting to retrieve a Horcrux, which is an object containing a part of Lord Voldemort's soul. Regulus became aware of the importance of the Horcruxes in Voldemort's plans for immortality and sought to thwart those plans. Understanding the dangers involved, Regulus took a brave stand against Voldemort by venturing to retrieve the locket Horcrux from the cave. Unfortunately, he was killed by the Inferi, animated corpses enchanted by Voldemort to guard the said Horcrux. His death was a self-sacrificing act to protect others from Voldemort's tyranny and further aid in the eventual destruction of the Dark Lord.