Petroleum Dragon

Breeding
In the game it is impossible to directly breed the dark and water elements. Therefore you must breed two dragons that each contain one of the two elements at the Breeding Mountain or Ultra Breeding Tree in order to get a Petroleum dragon.

Examples: Neon and Nenufar Venom and Blizzard

Damage Calculator
To explain, when attacking with Water, Petroleum deals double damage against Fire and Electric, medium damage against Water and Ice, and no damage against Plant. When attacking with Dark, Petroleum deals double damage against Water and Metal and half damage against Plant and Dark. When defending, Petroleum takes double damage from Fire and Dark, medium damage from Water and Plant and take no damage from Earth.