Form V Adegan Diatium Lightsaber was a custom built lightsaber using a Custom Form V LS Model, with a Adegan Crystal, and a Diatium power cell. It was used by Darth Vader and later was stolen by Galen Marek. It was created in 19 BBY by Darth Vader himself.
This lightsaber was originally powered with a Crimson Synthetic Crystal, making this lightsaber called the Form V Syntha-Diatium Lightsaber, but was (for an unknown reason) changed to Adegan. Probably in being that the Adegan represented strength over speed.
The Synthetic crystal was artificially created, rather than naturally formed by geological processes. Due to the methods used in their creation, they often formed with a crimson or red coloration, though they could be made with any color through special manipulations of either the creation process or the crystal itself, often through the Force.
After being stolen, Galen Marek possessed a Adegan crystal. Changing the color to aqua.
This weapon featured a thick, ridged handgrip, a high-output diatium power cell, a dual-phase focusing crystal, forward mounted adjustment knobs, and a beveled emitter shroud. However, due to the size of his mechanical hands, Vader was forced to make the grip considerably thicker and longer than his original lightsaber, resulting in an inelegant weapon, verging on ungainly.