Dekamemhnom was one of the most powerful Nightmare lords before he lost his kingdom.
Once lord of one of the most powerful nightmare kingdoms whose power was greater than the elemental kingdoms. His kingdom and power were derived from humanities fear of the plague. As the real plague disappeared over the years so did humanities fear of it thus over the years his kingdom lost power and disappeared until only he remained. Later on he became the king of an area of the World apart, it is because of the loss of his powers and kingdom that he is called the fallen lord of the plague. During the Black Travelers Journey through the world apart he infected and indirectly killed Kain. In the fight that ensued the meteors intervened and was captured by one of their members.
Though he has lost his kingdom Dekamemhnom remains extremely powerful, able to infect his targets with a disease that causes them to turn on each other. Not to mention he is very tough so much that after getting hit by Sealvia's most powerful attack he wasn't even damaged