In the novel 'Eragon', the protagonist barely managed to defeat Durza, the Shade who lead the attack on the Varden. Unlike the film version, the battle was largely a contest of wills - what the series refers to as 'a magician duel'. A moment's distraction by Durza allowed Eragon to slay him, but in retaliation he sliced open Eragon's back as he died. The wound was healed, but from that point on Eragon
was crippled and would often black out from agony when using certain muscles.
This continued throughout his training, and it appeared that no amount of healing or magic would fix the problem - at least not until
his transformation into an Elf/Human hybrid by the Menoa Tree.
It was often stated that Durza must have done something to make the wound as horrible as it was, but on the other hand it sounds like nerve damage to me.
Has author Christopher Paolini ever revealed the exact nature of this wound? Was it something like a curse or poison, or was it simply nerve damage from the spine being cut?