Inquisition is a technology in Age of Empires II: The Forgotten that is unique to the Spanish and can be researched at the Castle. Once researched, it lowers the time their Monks and Missionaries need for a successful conversion. The minimum and maximum times needed for conversion are lowered by about 1 second for units and about 7 seconds for buildings.
The Spanish already have good Monks as they can research all Monastery technologies. Inquisition furthers the Monk's and Missionary's capabilities, almost counteracting the effect of Faith if the enemy researches it. Without it, all enemy units except Light Cavalry, Eagle Warriors, and Teutonic units will be extremely vulnerable to a larger quantity of Monks.
In the Imperial Age, with Theocracy researched, groups of either Monks or Missionaries might even operate alone, converting enemy Villagers or buildings quickly enough to disrupt their economy and military production while conserving faith to respond to any retaliation. Missionaries can even kite enemy units while converting them, making responses with melee units even more inadequate unless in large enough numbers to overwhelm the Missionaries quickly. A player attempting to use Monks offensively for conversion purposes should keep in mind that it requires good micromanagement, since Monks do not convert enemy units unless they are attacked by them or directly ordered to do so.
The Holy Inquisition was an institution within the Catholic Church tasked with combatting all manners of heresy. Initially founded in the late 12th century AD to persecute Cathars and Waldensians, it rapidly grew in prominence in the late 15th/early 16th century when the Pope's authority was challenged by the newly-formed Protestant movement. The Inquisition was a powerful tool utilized by the Counter-reformation movement that aimed to contain the spread of Protestantism within Medieval France, Castile, Aragon, Portugal and the various Italian states and principalities.