Star Trek: 10 Character Fates Worse Than Death
9. Having Someone's Katra Shoved Into Your Brain
In Star Trek II: The Wrath of Khan, the crew of the Enterprise engages in open combat against a ship being controlled by Khan Noonien Singh, a character who went up against Kirk 15 years earlier in an Original Series episode titled "Space Seed." Since then, he was marooned on Ceti Alpha V, and he didn't hold Kirk in high regard.
The two battle it out, and ultimately, Kirk defeats his nemesis, but his victory comes at a price. The battle damaged the Enterprise's warp drive, and it was needed to get the ship out of the nebula, so it could survive the Genesis Device, which was initiated by Khan in a last-ditch effort to kill Kirk.
Spock rushes to the engine room, but McCoy attempts to stop him. Spock quickly gives his pal the ole' Vulcan nerve pinch, and then performs a quick mind meld, saying only, "Remember."
Spock fixes the drive, he dies, gets shot onto the newly-created Genesis planet, and is ultimately revived, but McCoy had a hard time dealing with the whole situation. When Spock did the mind meld, he forced his Katra into McCoy's mind.
Essentially, he uploaded his mental self into McCoy's brain, which did not handle the transfer well. It drove him mad and nearly killed him. While he was afflicted with Spock's Katra, death would have been a favorable release.