Imagine being able to adapt to any form of injury so that you cannot be wounded or defeated the same way twice. With each injury you receive, the more invulnerable you become. When enough time passes, you eventually become an unstoppable force who is immune to all forms of assault. The most prominent examples of this power I can think of are Darwin from X-Men and Doomsday from DC Comics. In my superhuman series, one of the main characters will have this power and it will change them in strange ways.