actually, it is magic damage, NOT physical damage. Magic damage reduction lowers the damage to your life, but not to your mana. It burns 50 mana, and deals 50 spell damage. (this holds true for all mana burning abilities) Mostly, this is a mid game skill. Early mid, to late mode/early end. Very early, it is alright. Before 10 it isnt that good, because creeps have low mana. After 10, most creeps have mana, so the bonus damage is really nice. After level 35 or so though, It starts to go downhill again.
An average hero attacks 2X a second. A duel lasts 60 seconds later in the game (especially against mana shield users) 70 extra mana burned a hit, ~120 hits, ~8400 mana burned/damage dealt from this skill alone. This of course goes on top of any other damage you are dealing. This is especially good against people who rush shoulders, or are about to make shoulders. They have low armor; so draining they high mana faster gives you a stronger chace of winning.
This skill isnt amazing, but it isnt useless. It is going to get a slight buff, but not a very big one. This skill is pretty decent as-is. Just not competitive enough for an offensive skill slot.
