"Kreacher drank, and as he drank he saw terrible things ... Kreacher’s insides burned ... Kreacher cried for Master Regulus to save him, he cried for his Mistress Black, but the Dark Lord only laughed ... He made Kreacher drink all the potion ... He dropped a locket into the empty basin ... He filled it with more potion. And then the Dark Lord sailed away, leaving Kreacher on the island ..."
-- Kreacher (HBP10)
Voldemort announces that he requires the services of a house-elf. Regulus Black volunteers Kreacher, the old house-elf who serves his family in Grimmauld Place, ordering him to do whatever the Dark Lord demands, then to come home. Voldemort takes Kreacher to the sea cave where he has prepared an island in the middle of an inferi-filled lake. On the island, he orders Kreacher to drink the potion in a stone basin. While Kreacher suffers unbearable pain and cries out for help, Voldemort places his locket horcrux in the basin, which then refills with potion. Leaving Kreacher to die, Voldemort sails back across the lake in the only boat. Kreacher tries to drink water from the lake to ease his suffering but is dragged under the water by the inferi. At this point, Kreacher follows Regulus’ orders and Apparates back to Grimmauld Place.