Flame Sun Blood is a passive skill. Any enemy that causes melee damage to the Holy Temple Knight will suffer retaliation from the Golden Flame, inflicting damage equal to 5% of the inflicted damage per level, reaching 75% at Level 15. Although this damage is considered both magic damage and flame damage and can be offset by various resistances, it is enough to give the opponents of the Holy Temple Knight a headache. It's worth noting that the Holy Temple Knight is a tank-type profession; you might end up receiving more damage than you inflict.
