Simon's armor (or sword) give him somekind of immortality through regeneration, so when the MC (Yoke or York, who knows) go back to the dungeon to rescue Jamie, he and the girls fights Simon, but since that mf literally becomes somekind of a monster, the MC finally decides to "kill" him, so he puts somekind of poison curse on him, that would only stop from melting him from inside out when he dies, but since he's now immortal, he enters in an eternal loop of agony.
And the cherry on the top of the cake is: the place where they fight is full of shadow monsters, so after the fight, those monsters will have a nice and eternal piece of meal named Simon the cursed immortal.