This skill works pretty simple. It's a buff-skill. You cast on any enemy hero you want and he will take upon casting an extra % of mana.
1 level 75 mana, 30 seconds cd, skill takes extra 30% of skill mana cost, lasts for 6 seconds
2 level 85 mana, 30 seconds cd, skill takes extra 40% of skill mana cost, lasts for 7 seconds
3 level 95 mana, 30 seconds cd, skill takes extra 50% of skill mana cost, lasts for 8 seconds
4 level 105 mana, 30 seconds cd, skill takes extra 60% of skill mana cost, lasts for 9 seconds
For those who didn't quite catch what this skill is about, an example:
You are fighting against Lina. You place this buff on her. She's casting her combo:
Light Strike Array + Dragon Slave + Laguna Blade.
So, originally this would cost(max level of all skills of course):
125 mana + 140 mana + 680 mana.
Now, with this buff this will look like:
200 mana (125x60%) + 224 (140x60%) + 1088 (680x60%)
This skill has something common with Ezalors mana leak, but in case with mana leak you still can stand and cast your skills like no problem. But with this buff you will think twice before casting. I consider this skill highly effective against nukers/casters.
This kind of skill would be useful on a mana-burner type hero. At level 4, 60% is quite large percentage of extra mana required for casting. Especially if the mana cost is high. For some reason, I find this spell extremely effective against Storm