Serving in the 1st Sothan Irregulars, he was a Sergeant stationed to help guard the Pharos on his homeworld. Mercius was said to have been named after a great nation in Terra's past and was personally close to the closely-knit community of the Sothan Auxilia. During the Battle of Sotha, Mericus led his squad in a desperate defense against the Night Lords with the aid of Ultramarines Scouts led by Oberdeii. Eventually however Mericus and his squad were captured by Night Lords under Gendor Skraivok. After being questioned on the location of a hidden entrance to the Pharos which he refused to answer, Mericus was tortured and killed by Kellenkir.