Permanent injuries may occur, at the discretion of the GM, whenever a player character takes their max HP in damage in a single blow. These injuries may manifest themselves but only in the nature in which the damage was received.
For instance, a wyvern gnawing off a character's arm doesn't mean their permanent injury is permanent blindness. It would make more sense for their permanent injury to be a missing arm.
These injuries are entirely circumstantial, subjective, and ultimately up to the GM. Characters with permanent injuries may also incur penalties to certain roles, also at the discretion of the GM.
Permanent injuries may also be placed on a character in situations where it makes sense to impose them.