wotc
If a red instant or sorcery spell you control causes damage to be dealt, that spell will always identify the source of the damage. In most cases, the source is the spell itself. For example, Lightning Bolt says “Lightning Bolt deals 3 damage to any target.” Such a spell is affected by Fire Servant’s ability. If the source of the damage is something else (for example, if the spell is Soul’s Fire, which says “Target creature you control on the battlefield deals damage equal to its power to any target”), it isn’t affected by Fire Servant’s ability.
