Pendant of Celerity is dropped by Lord Valthalak in Upper Black Rock Spire. There is a 25% Drop Chance. Lord Valthalak has to be summoned as part of the Mea Culpa, Lord Valthalak quest. This is a quest chain starting with More Components of Importance. The quest giver is Bodley, who is found in Searing Gorge. Lord Valthalak is summoned where you fight The Beast.
Lord Valthalak Location