Inferius

... um-

But we also get a description of the Inferi dragging him below the lake.

Yep.

Right?

So then you could argue that, like, we know that would happen either way, 'cause obviously Harry's not, you know...

Like, they appro- they do that when Dumbledore and Harry are there.

Um, so you could argue that it is the vow enacting itself, or it's like, "I'm going to die because of the vow either way, but before the vow takes him, the Inferi just pull him under anyway."

Oh, yeah.

And the subject matter is important.

You know, Snape can nitpick, but they actually have to know this because Seamus has asked about Inferi.

And after insulting people for a while, Snape says the Inferius is a corpse that has been reanimated by a dark wizard's spells.

It is not alive.

It is merely used like a puppet to do the wizard's bidding.

He realizes that the location had personal significance to Voldemort and eventually works out that something important has been hidden inside, but finding the cave was just step one.

From there, Dumbledore had to locate a magically concealed entrance, open it using blood, cross an underground lake filled with Inferi, and then drink a potion that leaves him confused, terrified, and physically helpless.

By the time Harry and Dumbledore escape, Dumbledore is barely capable of defending himself, and after all that, the locket they retrieve isn't even the Horcrux because Regulus Black had already taken it.

"Dumbledore began to cower, as though invisible torturers surrounded him.