In Advance Wars: Dual Strike, the Black Crystal is a Black Hole building that can restore 2HP and restore Fuel and ammo to Black Hole units in a radius of two squares away each turn. The Black Crystal cannot be used in design mode. Like most buildings, the Black Crystal has 99HP. Destroying all Black Crystals wins the level.
The Black Crystal was an idea of the Bolt Guard and rumored to be created by Lash. The Black Crystal sucks up Omega Land's energy for Von Bolt, Kindle, and the Black Hole Units around it. The higher version of the Black crystal is The Black Obelisk. The terrain around the Black Crystal will eventually turn into a desert. If it drains too much energy or too many Black Crystals are in one area, the land will turn into a wasteland, turning the water red.
The Black Crystal appears in the missions Victory or Death!, Healing Touch, For the future!, and Means to an end! The objective in the former two are to simply destroy the crystals to win the mission, however, in For the Future, there is also a Black Obelisk that needs to be destroyed. In Means to an end, the Black Crystal is located on the second front.