Help with Harry Potter and the Deathly Hallows?

It was a trap for Snape in case he ever attempted to return to 12 Grimauld Place. It speaks with Moody's voice because he was likely the one who set it up. The figure takes the form of Albus Dumbledore, which is why Lupin addresses it as Albus (presumably, what he says is what activates the counter-charm to banish the figure back into dust).