A couple nights ago, I was watching a movie about Tristan and Isolte, and it reminded me that several legends include the idea of wounds that never heal. So what about a magic sword that inflicts grievous wounds on a critical hit? That strikes me as an interesting variant on "+1 sword"-style magic items.
For that matter, you could just define any critical hit from an enchanted being as a grievous wound. Until sufficiently powerful magic is used to heal the wound, the victim can't heal at all. It makes combat very scary, however.