Looking through ROT, the practice of Ynfaenal Magic or Necromancy would seem top corrupt the soul or body but there doesnt seem to be any mechanics for this. Perhaps, borrowing a little from Warhammer 3rd ed, their needs to be some form of Corruption. Corruption points could be gained from encountering corrupt areas or using cursed items, or as a byproduct of casting Ynfaenal or Necromantic magic. Perhaps 1 is gained for every 3 Threat or 1 Despair on a spell casting check. Once a character gets Corruption points equal or greater than their Wound Threshold (for Ynfaenal corruption) or Strain Threshold (for Necromantic corruption), they have to roll a d100 to determine what mental or physical mutation occurs, clearing Corruption once this is done.