Must have mental illness/phobia, Ideas?

[Misereor]

Backstory hook: Sudden invasion of left-handed half-Merfolk paladins named after the great merman hero Slartibartifast!

In our teenage years, one of my players created his own character class, using the rules in the 2nd ed. AD&D DMG. He had some points left over that he didn't know what to do with, and ended up spending them on "+2 Save vs being hit by flowerpot on rainy Thursday". Naturally, their arch nemesis for that campaign became an insane horticulturalist, with the specialty of summoning animated construct: (flowerpot), a penchant for weather control, and some religious justification for conducting his evil plans for world conquest on Thursdays.

@subject of thread
This page has some nice phobias:
https://blog.oxforddictionaries.com/...ional-phobias/

I particularly like Luposlipahobia; the fear of being poursued by Timber Wolves around a kitchen table, while wearing socks on a newly waxed floor.