Once a murder has happened, to go back in time and prevent it might seriously mess with the timeline, and that is where the Time Patrol's Murder Squad comes in. They try not to kill anybody themselves, except in emergency defence of themselves or the timeline, but they are known as the Murder Squad because they are there to make sure the murders of the past happen.
Created in the far future, then sent through a temporal rift to the ancient past by the Chronomage Cara, the Circlet is an artefact designed to focus a mage’s energies when attempting to manipulate time.
It looks like a cook's book. It is a cook's book. It is also where the cook, who happened to be a master spy, encoded all his secrets and contacts. The book has innocently fallen into your group's hands. Everyone wants it, and many are not subtle or peaceful in their attempts.