We all know of Ollivander by now, of course: he's the dude who sells all the kids their wands before starting at Hogwarts. Or at least, that's what he did before his shop was destroyed a couple of movies ago.
When this film opens, he's still regaining his strength after being trapped in Malfoy Manor as Voldemort's prisoner. Despite the fact that he's still weak, Harry wants to chat with the wandmaker about the legend of the Elder Wand, a supposedly unbeatable wand that's also one of those Deathly Hallows mentioned in the title:
HARRY: And what do you know about the Deathly Hallows?
OLLIVANDER: It is rumored there are three: The Elder Wand, the Cloak of Invisibility, to hide you from your enemies, and the Resurrection Stone, to bring back loved ones from the dead. Together, they make one the Master of Death. But, few truly believe that such objects exist.
HARRY: You know one exists. You told him about it. You told him about the Elder Wand, and where he could go looking for it.
OLLIVANDER: He tortured me. Besides, I only conveyed rumors. There's... There's no telling whether he will find it.
HARRY: He has found it, sir. We'll let you rest.
OLLIVANDER: He's after you, Mr. Potter. If it's true, what you say, that he has the Elder Wand, I'm afraid you really don't stand a chance.
So, yeah, unfortunately Voldemort tortured Ollivander, and the old wandmaker gave up some intel that he thought could never be useful to the Dark Lord…but which led him right to he most powerful wand in the world. Obviously he didn't do it on purpose, but that's a pretty big boo boo—and one that's going to be pretty dangerous to Harry.