It appears that the Wolf Knight is dealing true damage on skull hits. This definitely happens after it has cast its spell at least once; not sure if it happens before. Only observed it in PvP (iOS).
E.g., Enemy Wolf Knight casts Hunt spell and hits my troop for -10 true damage, leaving troop with 20+ Armor and 4 Health. On its next turn, Wolf Knight (Attack 10) gets a skull hit and kills my troop (-4 remaining Health) instead of hitting armor.