A Daemon Engine or is a piece of machinery that has been corrupted and consumed by Chaos so that it serves as a type of host body for a demonic entity. Although more than vehicles can be corrupted by Chaos, Daemon Engines are specifically vehicles and, on rare occasions, some types of large mobile weapon such as cannonry. A perfect and common example of a Daemon Engine is the Chaos Defiler.
Creating a Daemon Engine requires that the entity be summoned by one who has knowledge of such things, namely a sorcerer. The Daemon is summoned inside the hull of the machine so that the corruption spreads throughout it's circuits, gaining complete control of all funtions.
Mutation Over Time
Like most Chaotic corruption, change may happen at varying speed depending on the strength of the daemon trapped in the machine's hull. In any case, mutation due to possesion always begins internally and spreads outward. Interior funtions and control are the first to be taken over. Exterior change only happens last, if at all.
Characteristics
The most notable characteristic of a Daemon Engine is that they have minds of their own rather than being piloted. Daemon Engines have an insatiable bloodlust and revel in violence and destruction. Often they will destroy anything that lies in their path regardless of allegience and so must be chained up until they can be unleashed upon the enemy.
Like all entities of the Warp, Daemon Engines exude an aura of death and hatred that can overwhelm a lesser man. Untainted humans near one of these beasts often fall to their knees in horror and the demons presence fills their minds. Even the warriors of the Adeptus Astartes can only just withstand this metal barrage.
Psykers stand the best chance at countering these monsters. Their presence and power can reduce the daemons hold on the Materium and prevent the demon's darkness from attacking others.