The Tome of Ectoclades

WATCH MASTER or CAPTAIN model only. Once per battle, after you have selected your Oath of Moment target, the bearer can use this Enhancement. If it does, select a second enemy unit to be an Oath of Moment target. 

Designer’s Note: This means that each time a model with the Oath of Moment ability makes an attack that targets either of your Oath of Moment targets, you can re-roll the Hit roll.