Tenebraid

Tenebraid is a duel between Mages. Your opponent and you both begin with randomly-generated spells. To achieve victory, you must reduce your opponent's health to 0, but victory is only a secondary objective. 

Every time you attack your opponent's Shadow, there's a chance you'll plunder one of their spells and merge it with a spell of your own to create something new and potentially much, much more powerful -- and also unpredictable. Once you create a  new spell, your opponent's shadow stabilizes to full power and you can attack again, if you wish. 

Utilize an array of defensive spells to protect yourself while afflicting your opponent with debilitating conditions to slow them down, and watch the flow of the duel carefully. Do you need to defeat your opponent quickly? Can you afford to plunder their shadow for new spells in anticipation of a more powerful foe? Should you end the fight quickly to summon another enemy with new spells for you to steal? The decision is yours. 

The game's Help page provides important information about the mechanics. You can hover the mouse button over a spell's name or over a player condition to show its effects. 

Tenebraid Game Design Document