Y'Shaarj (pronounced Yah-Sha-Raj) is an Old God, one of several malefic beings that were sequestered by the Titans during Azeroth's primordial ages. Located in what is now Pandaria, the mantid race worshiped his seven heads. The Titans slew Y'shaarj, but with his dying breath he cursed Pandaria by infesting it with "shadows of his former self".
The mantid seem to believe Y'Shaarj may return someday, and if so, they will ally themselves with him.