Alert: Really broad idea.
I want a character who in some way devotes his life to making himself unassailable, invincible. This could be a fantasy ‘invulnerable’ or a real world case have having garnered wealth and favors enough that you can get away with murder. The goal is accomplished, but then the character falls. Why does the character fall? Because he’s done. Finished. It’s like reaching the end of the Internet. There’s nothing to do after that point, so he just collapses inwardly. His only goal was to make himself invincible, and in doing so he killed himself.
Metaphorically, I think. Obviously I’m still trying to wrap my brain around this one.