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 the Castle Age is reached. Once researched, it increases their Monk and Missionary conversion rate.
Strategy
The Spanish already have good Monks as they can research all Monastery technologies. This technology furthers the Monk's and Missionary's capabilities, almost counteracting the effect of Faith if the enemy researches it. Without it, 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.