Xyvareth is the Naiad King, crowned thousands of years ago. He controlled the Corruption's power throughout his reign. He first appeared in comic #210.
After a while, the Corruption seduced him into turning against the other Species, starting a thousand-year war between the Dryads and the Naiads. The Dryads had their opposite source of power, the Hallow . No-one could win, as the main Species were created equal.
In the middle of the war against the Dryads , Xyvareth heard about a ritual that would lock away the Hallow forever and leave the Corruption to triumph. This ritual involved infusing a Light's Bane with the four sacred swords of each Species; the Blade of Grass representing the Dryads' element, Earth; the Gossamer Sword representing the Faerie's element, Air; the Fiery Greatsword, representing the Djinn's element, Fire; and his own Muramasa, representing the Naiads' element, water.
The combination of these four swords created the strongest source of dark power in the world - the Night's Edge (also known as the Edge of Night ). Xyvareth took his servants to the Underworld and summoned an ancient demon (possibly referencing the Wall of Flesh ) by sacrificing his own counsel. Using the Night's Edge , he bound this force and commanded for the Hallow to be locked away.
However, once the ritual was completed, both the Hallow and the Corruption disappeared, along with the Naiad and Dryad empires and main powers. No one knows what happened to Xyvareth or his Night's Edge , but if a new one can be made or someone can find it, the power of this demon can be stopped once and for all, and bring back the Hallow.
He was supposedly followed up by, Xylan and Xyvgane.