Spell Advice

I'm making a spell sort of like orb of lightning for a hero in a campaign, and I need help.
The spell will be a melee autocast hero spell, and add bonus damage plus stun the enemy unit when used. It will cost a certain amount of mana. Unfortunately, I can't use Purge because it does no damage and can't autocast, and I can't use Orb of Lightning because there's no field for mana cost. What do I do?

Bash might work, but you want it to have a mana cost... I have an idea.

E: Unit Learns "Bash"
C: Unit = X
A: Set (Variable) = 1

E: (Unit X) Is ordered to attack ....................[Need to find an On Hit type thingy so It wont bug and steal mana from the unit]
C: (Variable) = 1
A: Set UNIT Mana --> UNIT Mana -10

Does this work?

I opened bash, and added a mana cost and cooldown, but it works all the time! Is there a way to edit so that Bash will have a working cooldown?