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Why did Dumbledore drink the potion? How did that help to find the horcrux?

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It was part of a protective scheme for the horcrux, designed by Voldemort.

Harry and Dumbledore went to the cave in the cliffs, and found a lake with a central island. When they crossed to the island, they found a stone basin. Dumbledore worked out that the horcrux was at the bottom of the basin, but it was impenetrable by any means except drinking:

“You think the Horcrux is in there, sir?”

“Oh yes.” Dumbledore peered more closely into the basin. Harry saw his face reflected, upside down, in the smooth surface of the green potion. “But how to reach it? This potion cannot be pen-etrated by hand, Vanished, parted, scooped up, or siphoned away, nor can it be Transfigured, Charmed, or otherwise made to change its nature.” Almost absentmindedly, Dumbledore raised his wand again, twirled it once in midair, and thencaught the crystal goblet that he had conjured out of nowhere. “I can only conclude that this potion is supposed to be drunk.”

Half-Blood Prince, chapter 26, The Cave

This was a deliberate defensive measure by Voldemort. The boat to the island was enchanted so that only a single person could reach it:

Dumbledore chuckled. “Voldemort will not have cared about the weight, but about the amount of magical power that crossed his lake. I rather think an enchantment will have been placed upon this boat so that only one wizard at a time will be able to sail in it.

“But then—?”

“I do not think you will count, Harry: You are underage and un-qualified. Voldemort would never have expected a sixteen-year-old to reach this place: I think it unlikely that your powers will register compared to mine.”

Half-Blood Prince, chapter 26, The Cave

So only one person can get to the island, and when they get there, they have to drink the poison. Assuming that doesn’t kill them, the Inferi in the water almost certainly will. This is a pretty good defensive strategy, as long as only one person actually does get to the island.

As usual, Voldemort was undone by his arrogance: believing that a young person would never reach the cave, or that a Squib/Muggle wouldn’t accompany them (perhaps carrying bottled water, which might be immune to his enchantments to evaporate all other water).

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