Princeps

A Princeps (pl. Princeps) is the senior officer in command of one of the towering Imperial war machines known as Titans, and all of its crew men are subordinate to him or her. Princeps are all men and women of exceptional willpower, which is required to enter a mind-link with a Titan maintain their sanity intact amidst the roaring spirit of destruction housed within a Titan's Adamantium hull. Although they belong to a separate military hierarchy of their Titan Legion and the Collegia Titanica, the Princeps are part of the Cult Mechanicus, and as such defer to the orders of powerful Adeptus Mechanicus Magi, who command the might of the Titan Legions and are able to issue them direct orders and decide on their assignments.

The Princeps exerts command over his war engine by the means of a Mind Impulse Unit -- a technologically complex device which allows for a cybernetic connection to the Titan's Machine Spirit, and impose his or her will unto it. This process is very difficult and dangerous, as Titans are possessed of ferociously potent Machine Spirit, one, unlike any other, harbouring its own savage and instinctive intelligence. The Princeps must break the machine's opposition and bind it to their will -- a feat requiring enormous willpower. Because of this, Princeps are immensely rare individuals, and one of the tasks the Collegia Titanica carries out is to discover them wherever they can be found and train them to command the colossal war engines. Whilst normally offices of the Adeptus Mechanicus search for their personnel within the Cult Mechanicus, the sheer rarity of such individuals enforces searching in the broader populace of the Imperium. Barely one in ten million possess the necessary qualifications, and even fewer of this number make it through the training to finally command a war engine. Because of this Princeps are immensely valuable individuals, and their home Forge World will spare no effort recover them should they go missing in action, and a merest chance exists that they are still alive. There are two ways in which one can become a Princeps -- undergoing training in Collegia Titanica academies, as mentioned above, or advancing from the rank of a Moderatus to a full Princepture, which is usually the command of Warhound-class Titan.

When the Princeps is linked to a Titan's MIU, he feels the Titan's body as if it was his own -- his hands become weapon arms, his senses are turned into powerful auspexes and his skin becomes the thick Adamantium hull -- an experience of supremacy and escape from human frailty that addicts every Princeps to link with a Titan. The Princeps is then able to command the Titan intuitivelly -- he merely needs to think of undertaking an action and the machine complies. During this, the Princeps does not lose conciousness of his own body, and is able to simultaneously move and act as normal.

Relationship with the Machine Spirit
When a Princeps first links with a new Titan, he or she must break its resistance and force it to acknowledge its new master. Once that is done, however, the Machine Spirit's resistance does not cease, and each reconnection, altough longed for because of the unique feeling it delivers, is painful and mentally taxing for the Princeps. Being linked to a Titan will, either through the sheer mental strain of connecting to and becoming a being that dwarfs a human in nearly every respect, or through the actual patient endeavour of the Titan's Machine Spirit, eventually result in the Princeps losing sanity, being broken in turn Titan, or being completely overwhelmed by the domineering Machine Spirit, lost in the machine's mental landscape -- at which point disconnecting the Princeps from the Mind Impulse Unit will result in certain death.

Being linked to the Titan's Mind Impulse Unit is a considerable effort in itself, and controlling the machine's systems precisely is difficult. Most Princeps are incapable of controlling all of a Titan's functions simultaneously due to the sheer, unbearable mental tumult it inflicts, and those who do only so for short periods of time. For this reason, Princeps are assisted in operating these towering war machines by junior officers called Moderati -- whose role is to operate some of the Titan's systems in their commander's stead -- lessening the strain placed on the Princeps and allowing for the efficient handling of the machine.

The mind-link is a very intimate and immersive connection, causing the Princeps and the Titan to leave an imprint in each other's consciousness and grow increasingly alike over time. The Titan's Machine Spirit will learn from its Princeps and from the battles they take part in, storing the knowledge and making it available at all times and to any Princeps that will command the machine in the future. The Princeps on the other hand, will, with time, begin to show traits of character akin to those of his or her war engine. The nature of the Machine Spirits varies between individual Titans, however war engines of the same class will be similar. Thus, the commanders of Warhound-class Titans are known to be bellicose and belligerent, and sometimes loners, though the latter may be due to their generally solitary assignments as scouts, while the Princeps of other war engines are more cool-headed and better capable of cooperation as part of a larger force.

Chaos Princeps
During the Horus Heresy, half of the Titan Legions declared support for the Warmaster Horus, and since those ancient times, the fickle blessings of the Ruinous Powers have warped many of them. Although some of the traitor Princeps have remained largely unchanged, and are presumed to operate their machines as described above, the rest have become hideously disfigured and merged with machines they command, linking with them not only in mind, but in flesh as well.