He is supposed to have a piece of Unholy shrapnel in his body that can't be removed with severe complications. Although it rather depends on the size of the bloody thing, it's suppose to weaken him as the years go past.
Now the question: where would it be located so that it can't, at any cost, be removed, without severe complications? It really is not supposed to be removed at all and the piece of shrapnel is cursed with Unholy magic, so he's going to be weak as hell. (And we are not on Earth, but in a realm that is similar to those of the Ancient Greeks, so no surgery either.)
If this isn't clear, feel free to ask me to elaborate.