'Well, the class seem fairly advanced for their level ...Though I would question whether it is advisable to teach them a potion like the Strengthening Solution. I think the Ministry would prefer it if that was removed from the syllabus.'
-- Umbridge to Snape (OP27)
Dolores Umbridge was made the first Hogwarts High Inquisitor in history when the Ministry passed Educational Decree Number Twenty-three (OP15). She was given the authority to inspect her fellow teachers and, should they prove lacking, have them sacked. She took the job with relish and clearly enjoyed bullying others.
Wizengamot elders Griselda Marchbanks and Tiberius Ogden resigned as a protest to show they were behind Dumbledore instead of Umbridge (OP15).
Umbridge used her power as High Inquisitor to harass teachers in the classroom, eventually ousting Dumbledore and making herself Headmistress when her "Inquisitorial Squad" found Hermione's sign-up parchment for Dumbledore's Army in the Room of Requirement (OP27).
The job of High Inquisitor suited Umbridge's sadistic personality and gave her free reign to mistreat and punish others, especially children. It was the classic case of a little power going completely to someone's head until she became a tyrant.
Dolores's appointment as Inquisitor at Hogwarts gave full scope, for the first time in her life, for her prejudices and her cruelty. She had not enjoyed her time at school, where she had been overlooked for all positions of responsibility, and she relished the chance to return and wield power over those who had not (as she saw it) given her her due. . . . She actively enjoys subjugating and humiliating others, and except in their declared allegiances, there is little to choose between her and Bellatrix Lestrange.(Pm: Dolores Umbridge).
The author went even further in her personal comments about Dolores:
Her desire to control, to punish and to inflict pain, all in the name of law and order, are, I think, every bit as reprehensible as Lord Voldemort's unvarnished espousal of evil.
So there you have it - Umbridge is a character somewhere between Bellatrix and Voldemort on the scale of evil.
Inquisitor = Latin for "searcher, examiner" - the title of an officer of the Spanish Inquisition
The Spanish Inquisition was a tribunal of the Catholic Church in 14th century Spain. The Inquisitors were charged with punishing anyone thought to be Jewish, Protestant, or merely disloyal to the throne of Spain. Sometimes there were literal witch-hunts looking for those with pagan beliefs. If someone was convicted they would be publicly tortured or burned at the stake in a mass execution known as an "auto-da-fe" (Wikipedia).
There was a Monty Python sketch titled "Nobody expected the Spanish Inquisition" making fun of sadistic Inquisitors who enjoyed torturing others or being tortured themselves. The phrase has become common in popular culture when someone feels harassed by questions or personal attacks.