So how it works is similar to the Biblical curse "The Mark of Cain". However, since no RP will really allow something so mind bogglingly OP as returning all damage towards the sender times a factor of 7, 24/7, I'm thinking of changing the effect to work in one of two ways.
First, the curse is focused on ones arms and hands, and each arm uses it in a slightly different way. One creates an invisible field in whatever direction one is putting out said hand, catching all incoming attacks within a front size of 5-7 feet, then sending it blasting off at 7 times the force in the opposite direction, while the other arm would stop any attack that would collide with it when the curse is active and allow the user to hold it in for 10 extremely painful seconds at most, before either unleashing the attack into a nearby object/person via punch or having his arm be subjected to it instead.
The second way involves the first two ways, but they are in affect around the whole body. It also can activate automatically to a sudden threat, before the user might even be aware of it(say, someone tried to shoot them with a sniper rifle), if the ability isn't on cooldown.
Limits would be that using this ability is pretty taxing on ones energy and stamina, a 30 second to 1 minute cool down(this is per arm on the first one), and that some damage is likely to go through the longer you hold it in(this is for the second arm effect).
