The Steel Inquisitors form the Canton of Inquisition. An Inquisitor is the ultimate fighting machine. Bestowed Hemalurgically with all the Allomantic powers commonly known during the Final Empire as well as some of the Feruchemical powers, they are some of the most powerful creatures in the empire.
Among the hemalurgic spikes that an Inquisitor receives, two are driven through the eyes of each Inquisitor. It is unknown whether this positioning is necessary for the effectiveness of the hemalurgy, or whether the Lord Ruler simply wanted to inspire fear in the public by the gruesome effect this had on the Inquisitors' appearance. Regardless, this left the Inquisitors without eyes, but still able to see through constantly burning Steel and/or Iron. This burning, hemalurgically enhanced, allowed them to "see" even the faintest traces of metal: in the ground, in buildings, and even in people's bodies. This gave them the ability to see, after a fashion, almost everything that normal people saw, though with certain limitations and enhancements.
The Lord Ruler built weaknesses into all his creations, including his Steel Inquisitors. Inquisitors can be controlled by very powerful Soothing, leaving them vulnerable to Mistborn with duralumin. In addition, the Lord Ruler did not give them all the powers hemalurgy could bestow, limiting their abilities. Their largest weakness is the large spike between the shoulder blades of every Inquisitor. This spike ties together the two spikes through the brain and the spikes in their torso. If it is removed or the link is broken, the Inquisitor will die. Decapitation or crushing the head will break the link.
Despite these weaknesses, Inquisitors are very hard to kill. The significance of their central spike was kept a secret until Marsh revealed it to Vin by killing his fellow Inquisitors. They were capable of storing health, which allowed them to heal most battle wounds instantly. Many Inquisitors were already Mistings, mainly Seekers– some even Mistborn– and their hemalurgical spikes effectively doubled the strength of their allomancy.
Potential Inquisitors are selected from the members of the Canton based on merit and Allomantic ability. The group paid special attention to Mistborn or Seekers amongst the Canton, since such individuals would become capable of piercing Copperclouds upon their conversion. Piercing copper is vital to their objective of seeking out half-skaa Allomancers who think they're hidden by Smokers.
After the death of Lord Ruler, all surviving Inquisitors came under the control of Ruin as a result of their numerous amount of spikes. Unlike Kandra or Koloss, they had very little control over themselves under Ruin's influence, whereas Koloss could break away from him in a blood-frenzy, and Kandra could pull out their spikes if Ruin tried to control them. The Inquisitors worked toward its goal of destroying all life. To facilitate this, they were ordered to attack the remnants of the Terris people in order to create additional spikes charged with feruchemy. A few Inquisitors also gained additional allomantic powers associated with the previously secret higher metals, through the death of mistborn.