I'll point out first that when people use the term "inquisitor" they are normally referring to a functionary of the Spanish Inquisition.
In point of fact an "inquisitor" was a canonical term for anyone designated by a bishop (including the Bishop of Rome) to investigate some kind of irregularity.
Such inquisitors existed in the 600s and perhaps earlier. The first heretics ever questioned by clergy canonically constituted for the purpose were the Priscillianists in the 4th century.
There was an official inquisition sent to question the Catharists in the 1160s, for example - 70 years before the incident so obscurely referred to in your post.
Parenthetically, there was a group of Christians who emphasized Bible study and lay instruction at around the same time. They were called Franciscans - and they were not troubled, but officially supported by the Pope.
The Waldenses were investigated for other reasons.
What exactly was the inquisition about, if not hunting heretics?
The Spanish Inquisition was about examining whether persons accused of apostasy were, in fact, apostates.
And what does it have to do with Martin Luther?
You brought up Protestants in the first place and I pointed out that the Spanish Inquisition was created years before there were any Protestants.