Upgrading Units?
 

I noticed there is a button on my priests that lets you upgrade to inquisitors. It's the first unit I noticed that can be upgraded. However, I the button you click on is greyed out. How do you get to upgrade the unit? I already tried upgrading when there were very few priests, as I thought it may be the leadership difference in the units, but this didn't work either. Anyone know how this works?

It's the first and the last unit that can be upgraded, but sure it's worth it. You need some gold (150/priest?) and a Mind Ability named Inquisition, which is quite costly, because it requires Keeper of the Light and Tolerance.