New Dark Spell Idea

Name: Mirror spell, Thorn spell, Reflect spell, haven't thought of a good name yet, but these are the ones I'm thinking of.
Effect: Defenses in the area have a percentage of the damage they do dealt back to them.

Duration: Lvl 1: 5 seconds, plus 2/3 seconds each level with the maximum level gaining a second or two extra.

Damage: Lvl 1: 10% of damage dealt, plus 2/3/4/5% each level ( subject to balancing) with the max level getting a bit of a boost.

If this seems a little overpowered I thought of a plan for that too, make it also affect your troops. Any troops in the area of effect will also be chilling away at their health when they attack.

Tell me what you think ^.^