Titan's Lightning Bolt
This spell can be cast when the combination artifact Titan's Thunder is equipped. It has no spell point cost, and does 600 damage no matter the hero's Spell Power.
This spell can also be learned if you acquire the grail in the Conflux, teaching you all spells.
Comments
The most effective direct damage spell there is in terms of damage versus power.
If the special conditions affecting Lightning Bolt and Chain Lightning also apply to this spell is, to my knowledge, untested, but probable.