Warning Contains spoilers.
At the end of Harry Potter and the Half-Blood Prince, Dumbledore drinks the potion. Harry even says that the goblet scrapes the bottom of the basin.
Dumbledore drank like a child dying of thirst, but when he had finished, he yelled again as though his insides were on fire. "No more, please, no more ..."
Harry scooped up a tenth goblet of potion and felt the crystal scrape the bottom of the basin. "We’re nearly there, Professor. Drink this, drink it. ..."
He supported Dumbledore’s shoulders and again, Dumbledore drained the glass; then Harry was on his feet once more, refilling the goblet as Dumbledore began to scream in more anguish than ever, "I want to die! I want to die! Make it stop, make it stop, I want to die!"
"Drink this, Professor. Drink this. ..."
Dumbledore drank, and no sooner had he finished than he yelled, "KILL ME!"
"This — this one will!” gasped Harry. “Just drink this ... It’ll be over ...all over!"
Dumbledore gulped at the goblet, drained every last drop, and then, with a great, rattling gasp, rolled over onto his face.
Harry Potter and the Half-Blood Prince
Neither Harry nor Dumbledore filled the basin with the potion when leaving. Yet, in Harry Potter and the Deathly Hallows, Voldemort looks into a basin filled with potion once more.
"Potter,” said Professor McGonagall, turning to face him again with superb indifference to the Carrows’ predicament. "if He-Who-Must-Not-Be-Named does indeed know that you are here— "
As she said it, awrath that was like physical pain blazed through Harry, setting his scar on fire, and for a second he looked down upon a basin whose potion had turned clear, and saw that no golden locket lay safe beneath the surface—
Harry Potter and the Deathly Hallows
Edit: Also, Voldemort had to refill the basin with the potion the time he went to the underground lake with Kreacher.
"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 . . . "
Harry Potter and the Deathly Hallows
During the time between when Dumbledore drank the potion and Voldemort when to see if the locket was still there, who filled the basin with the potion?
Note: My question is not a duplicate this question. I am asking for who, not how.
