Dreadnoughts. Heavy walker units that provide fire support for the Astartes and demolishes enemy constructions. Dreadnoughts are basically life support sarcophagus for heavily wounded Space marines so they can keep on serving mankind for many centuries to come. Kept in stasis for most of their time. They are only awoken in times of need. Uttering their gratitude for once again spilling blood. Being inside a Dreadnought isnt like being inside a mech. Usually the marine inside is little more than a torso and a brain. Their Central Nervous System is connected to the frame of the Dreadnought. Basically making it an extension of the marines body. This is extremely jarring even to the enhanced mind of a Space marine.