Diana Wynne Jones Wiki

The Dright is the malevolent and self-centered ruler of Series Eleven -- one of the multiple worlds which is unique in having only a single variation.

The Dright is extremely old, having taken lives from many other people to boost his own power.  He is adept at altering the physical environment around him, and at impressing or frightening his subjects and visitors.

Christopher Chant comes to Series Eleven with Millie and Mordecai Roberts to try to do several things.  First, they are trying to retrieve the various lives of Gabriel de Witt, which have been dispersed by Ralph Argent using powerful weapons he smuggled from Series One, and collected in Series Eleven by the Dright as part of his study of multi-lived enchanters.

Second, Christopher wants to retrieve the soul of Mordecai Roberts, who in addition to having worked as a double agent for Ralph Argent, was also sent by the Dright to Chrestomanci's world to do the bidding of the Dright, who owned and controlled him.

By first identifying Mordecai's spirit name of Tacroy, and then by discerning which of hundreds of silver objects offered by the Dright to confuse him actually contains Tacroy's soul, Christopher succeeds in both his endeavors and leaves the Dright very irritated indeed.