The Inquisitor is a position within the hierarchy of the Temple. The role is only fulfilled under particular circumstances. A similar but more powerful version of this position, the Grand Inquisitor, is even more rare.
An Inquisitor's primary purpose is to seek out heresy. A Grand Inquisitor has the power to purge heresy. They are granted powers beyond the law, allowing them to prosecute anyone they see fit using the Temple's own judicial system. During a heresy trial, the Inquisitor can be called upon as the witness for the prosecution, while the Grand Inquisitor typically is the prosecution.