Mysterious Riddle

It stalks us by both night and day. Always the hunter, never the prey. You dare not sleep, nor look away. Poison stings, lead to decay. Shriveled skin, so cold and grey. When will it strike? You never can say. Bone white fortress, blood red sand, With corpses piled as high a man. If you wish to defeat it, you best have a plan. Sword and shield had best be in hand. Killing brave heroes, dead by the score, Roar, roar, comes the Manticore.