A lichbriarDr357 (5,000gp) is a parasitic plant that drains XP from an immobile victim every day (starts at 10, doubles every day after), killing them when they reach 0XP. Wish/miracle can reverse the effect, but that's still nasty.
Wishfern can be grown from a 25,000gp seed and grants one wish, but takes 1d100 years to bloom. It also dies when anything touches it unless they succeed on a DC 40 Profession (gardener) check. You can theoretically get infinite wishes by replanting the seed it leaves behind, if you're willing to wait around for centuries.