Before Melania Black Née MacMillan died she gave one final order to Kreacher: protect her three grandsons no matter what. Years later, Regulus went to the cave to steal Voldemort’s locket horcrux. After swapping the locket for a fake, Regulus was attacked by the inferi and dragged into the water, he ordered Kreacher to leave him and destroy the horcrux. Due to Melania’s order Kreacher cannot fulfill Regulus’s orders so he saves him and apparate a near dead Regulus to Sirius’s flat.