Dark v2 has the best aoe in the game and in total it costs 22.5k gems.
You can buy it from the Gem merchant at Logue Town.
Each move costs 4500 gems.
The moves are:
Black Vortex: Q move. Costs 300 Stamina
True Liberation: E move. COST: 300 Stamina
Dark Nebula: R move. COST: 300 Stamina
Dark world COST: F move. COST: 300 Stamina (best AOE move in game and gives 2x exp which makes it the best for grinding)
Dark Flight: G move. COST: 300 Stamina (Gives you the ability to fly)