The Red Crystals, or Energen Crystals are galactic crystals with an unlimited amount of pure energy that are harvested by Cranassians to build weaponry.
The crystals are actually sentient artifacts left by the Cranassians before their demise, containing their souls and carrying their will. The crystals will infect military weapons and take control of them, then using them to attack whoever constructed these weapons. After units that carry Red Crystals get destroyed, the Crystals fly straight out like a space pod fired from a cannon, unharmed, returning to protect the Cranassian Core Fortress. It is possible that the Raiden mk-II and the Raiden mk-II ß both contain miniature Red Crystals.
Red Crystals only appear in the main Raiden series, from the first installment to the current main Raiden installment. The final boss in Raiden DX first appears as a grey metal sphere, then transforms into the boss tank. This suggests that Red Crystals have the ability to reshape metal to any form they desire. In Raiden V, it is shown they have the capability to both store and modify chemical elements, using them to change the atmosphere's composition.