Dwarven Possessed

Service in the Bastion Elites is dangerous, though that does go without saying. Risk of debilitating injury or death is assumed; serving in any military group carries that, much less one facing such an extreme environment. But death is not the worst potential risk faced by the Elites. Certain sorts of Elementals, with particularly strong magical personalities, are capable of possession.

Elemental Possession

When one of these powerful elementals attacks, it does so with such a forceful magical attack, carrying a strength of chaotic will, that it is capable of overwhelming a mortal's mind. If an Elite suffers an attack from one of these, and is unable to fend off the will of the elemental (extremely difficult even before factoring in that it may be attacking physically as well), the elemental could take absolute control of its mind and body.
A dwarf possessed in this way feels their mind being chained, shoved into the back of its consciousness. Their body ignites into flames, the magic of their soul enflamed. The dwarf's comrades witness this possession with horror and must defend against another suddenly present enemy. A possessed that is not captured or killed will then be a lone horror, lurking within the Titanic Deeps, seeking out others to attack and potentially turn into their own thralls and minions. Throughout this all, the dwarf's consciousness is present and awake, watching through their eyes but unable to exert any control over the actions their body takes. 

Exorcism

It is possible for the elemental spirit to be driven out of the mind of the possessed dwarf. It requires the possessed to be captured (a daunting task in and of itself). Then an exceptional series of rituals must be performed to remove it, performed by specialized dwarven magic users. These rituals are dangerous in the extreme, both for the exorcist and the possessed. The outcome is generally one of three. First, the exorcism fails and the possessed breaks free of its bounds, attacking anyone present (usually the exorcist and some number of Bastion Elites present as guards). Second, the exorcism succeeds but the dwarf dies. Finally, the rarest outcome: the elemental is driven from the mind of the dwarf who survives. 
"Survival" from such an ordeal is relative. The dwarf likely witnessed some number of horrors from within their own mind, and the trauma from being shunted to the back of their consciousnesses is extreme, putting it lightly. Dwarfs that pass through an exorcism are never the same and rarely return to any semblance of their prior health. Bastion has a small quarter dedicated to the care and retirement of those who have suffered from elemental possession, and those that go through it are treated with absolute deference and respect, and every dwarf who knows of that quarter knows what it means to survive such an experience.