After the capture of Dementors, Lord Voldemort sent Bellatrix Lestrange, Lucius Malfoy, Fenrir Greyback and accolyte Isaac McNair to follow and investigate a meeting Auror Alastor Moody was going to have with Albus Dumbledore's contact from the Westminster Abbey Wizarding Security. Their mission was to retrieve information from the contact, kill him and if possible, kill Moody.
However, Dumbledore prepared a defense and sent auror Sirius Black along with Moody, in order to meet outside the Abbey with Mario Holiday, Albus' contact and Chief in Wizarding Operations of the Abbey. After receiving word from Holiday of an incoming attack on the Millenium Bridge and several Muggle places, the aurors were attack by the Death Eaters. So he could not be followed, Holiday mannaged to disappear, but Alastor and Sirius fought Bellatrix Lestrange and the rest of their group while defending themselves.
After a short duel, Bellatrix retreated leaving alone Malfoy and McNair. When Sirius was going to get Malfoy, McNair attacked the auror and almost killed him, only to be murdered by a curse from Alastor's wand that sent him against a wall with a violent explosion. After they gained the information, Moody and Black returned to the Order's headquarters, while Malfoy fled to tell Lord Voldemort the whole event.