Bonder

From UnknowableWiki

The Bonder is the wizard or witch who establishes the Unbreakable Vow. He or she must hold a wand over the two people making the vow, and tongues of fire will come out of the tip of the wand, sealing the Vow.

Bellatrix Lestrange was the Bonder of the Unbreakable Vow between Severus Snape and Narcissa Malfoy (HBP2).