Consider using with Avatar of Lyssa to increase your maximum energy.
Bonus damage is calculated based on how much energy you have after triggering this skill; therefore, if you have a maximum energy of 25 at the moment of skill activation, you will deal (25-5)*2= +40 damage.
At 16 Scythe Mastery, you will need at least 27 + 5 = 32 energy to gain the maximum benefit from this attack.