The Unbreakable Vow Revealed

At Slughorn's Christmas party, Harry notices Draco Malfoy trying to gate-crash. Snape takes Malfoy aside for a private conversation. Harry, using his Invisibility Cloak, follows and eavesdrops. He overhears Snape offering to help Malfoy, revealing he made an Unbreakable Vow with Narcissa to protect him and complete his task if he fails. Malfoy refuses Snape's help, insisting he has a plan and does not need interference. He accuses Snape of wanting to steal his glory. The conversation confirms Harry's suspicions that Malfoy is on a mission for Voldemort and that Snape is deeply involved, though his true loyalties remain ambiguous.