What can change the nature of a man?

 Death.

 

 That was easy.

 

 Mortals are not sturdy enough to withstand any force strong enough to cause meaningful change. Brittle, or friable. The force causes damage, and they are unable to recover. They bite it. The best you can do is partial death, leaving a hole with the rest alive and constant. If you think you saw a change, either their soul was killed and replaced by some possessing entity, or you were wrong. Cosmetic change masquerading as a profound alteration. Illusionary appearance of change. 

 Higher level immortals can develop entire scar bodies. Imagine being reduced to 2/3 of one eye, and making a full recovery, plus or minus the fact the rest of the body is scar tissue. Scar brain, scar skeletal system, scar limbs with working scar muscles. An immortal that hasn't embraced Death can survive literally any injury - unless that injury is so horrific they decide to embrace Death.
 Scars, however, are change. They will not be the same as they were before the injury.