In What Harry Potter Movie is...?

It's from the 6th movie - Harry Potter and the Half-Blood Prince. Snape is making the Unbreakable Vow to Narcissa Malfoy, with Bellatrix as caster, because Voldemort order Draco to kill Dumbledore as punishment for Lucius losing the prophecy and generally screwing up in the DOM in the last movie. With the vow, Snape promises to help and ultimately take over for, Draco in his task with the consequence of death if he doesn't.