Meltmassif was a substance that was predominant in the caverns of Cronal's Shadow Base on Mindor. It possessed a unique electrochemical property that could act as a transmitter. It was used in the construction of the Shadow Throne, Shadow Egg, and the shadow stormtrooper's armor. The crystals themselves were implanted into the brains of unwilling participants known as Pawns and the task of doing so occurred at the Pawning Table. It could melt on contact with an external object, and in some cases an organic being.
Mineral based sentient beings known as Melters used the meltmassif as their medium and communicated between it. They could materialize out of the very walls that formed the caverns. The Melters would then take any organic intruders and deposit them into the meltmassif. It appeared that the deposition of a being into the meltmassif allowed them to harness that being's energy output. The substance was also found on comets in the Outer Rim.
Cronal used his Sunset Crown to control the Melters, and therefor the Meltmassif. He used it as a weapon, employing it to create pits similar to quicksand pits, trap his enemies, and capture intruders.
R2-D2 found that it was coursing with electromagnetism.