In skript custom potions?

[Enderwolf]

I making a custom enchant and i want the player to be able to do double damage anyone know how to do this?

e.g Diamond sword does 7 damage with the custom enchants it does 14 damage

 

[Enderwolf]

Use "increase damage by <amount>"

Ok added this but it double the damage for the attack not the person being hit.
How would i make it so when the victim attacks he does double the damage?