The Erebus-class monitors were of 7,200 long tons (7,300 t) displacement, 405 ft (123 m) long, with a maximum speed of 12 kn (22 km/h; 14 mph) produced by reciprocating engines with two shafts, and a crew of 223. The ship's main armament consisted of two 15 in (381 mm) guns in a single forward turret. This turret had been built as a spare for Furious.
The original secondary armament of two BL 6"/50-caliber Mk.XV naval guns was soon replaced by eight QF 4"/45-caliber Mk.IX dual purpose guns in single mounts and two QF 3"/45-caliber 20 cwt anti-aircraft guns, also in single mounts. Between the wars, the 4 inch low angle guns were replaced by anti-aircraft mounts and the 3 inch guns by eight 0.50 in (12.7 mm) anti-aircraft Vickers machine guns in two quadruple mounts.
The class mostly served in the Naval Gunfire Support role.