In Star Trek: The Search for Spock, we all got to see first-hand (second-hand?) how crazy a single Vulcan katra can make a person act.
Now imagine if someone had more than one katra fused to their mind. A lot more. Try hundreds.
The Enterprise is sent to scan the area of where the planet Vulcan once lay. The destruction is still recent, and the remaining Vulcans don't want to do it themselves for obvious reasons. Spock takes it upon himself that it is his duty to go regardless of how he feels about the planet's - and his mothers - death and goes despite others' protests. (Jim's, McCoy's, Nyota's, etc.)
And then he begins acting strangely. As in, wandering the halls, not responding to his own name, and then not recognizing his own friends and calling himself by a different name. The 'nervous fit' is played off as stress, they do their scan, and fly away.
And then Spock looses it again. This time calling himself another name. And another. And another. Until even Jim has to relent, there is something seriously wrong with a Vulcan with Multiple Personality Disorder. Spock suffers other psychotic episodes, including auto-writing where he simply writes name and date after name and date, filling up whole PADDs with names he doesn't know. When these episodes break, they leave him very ill and tired.
If you can, make it so that if a Vulcan Healer tries to see what's wrong with Spock's mind, he cannot gain access because Spock begins forcibly rejecting him or something similar. (He's got so many other voices in his head he can't stand one more.)
Of course, this needs a happy ending! With all the Vulcan Katras being discovered and placed into a new Ark, or something similar. I am not the author here!
Big hugs and thanks in advance!