Deep Secret

Description for DEEP SECRET by Diana Wynne Jones:

Rupert Venables is a Magid.

It's a Magid's job to oversee what goes on in the vast Multiverse. Actually, Rupert is really only a junior Magid. But he's got a king-sized problem. Rupert's territory includes Earth and the Empire of Korfyros. When his mentor dies Rupert must find a replacement. But there are hundreds of candidates. How is he supposed to choose? And interviewing each one could take forever.

Unless...

What if he could round them all up in one place?

Simple!