They were named after the two bomb vessels sent to investigate the Northwest Passage as part of Franklin's lost expedition (1845–1848), in which all 129 members eventually perished.

Erebus was equipped with two 15 in (381 mm)/42 guns in a single forward turret mounted on a tall barbette to extend the range of fire to 40,000 yd (22.7 mi; 36.6 km).

The Erebus class were designed to outrange German heavy shore batteries and they were also fitted with highly effective anti-torpedo bulges on each side of the hull.

During the First World War, the Royal Navy developed several classes of ships which were designed to give close support to troops ashore through the use of naval bombardment.

The monitor's engines would not start reliably and were prone to stalling,[5] making it impossible for the ship to achieve even the minimum standard for the trial of four hours of continuous sailing at full power.

Some of the main modifications were an increase in the power supply to guarantee a speed of 12 knots (22 km/h; 14 mph) and a change to the angles and lines of the hull to improve steering.

It would have 8,450 long tons (8,590 t) loaded displacement,[9][i] with a maximum operational speed of 13 knots (24 km/h; 15 mph) produced by triple-expansion steam engines with two shafts, and a crew of 204.

[8] Orders for two vessels of the new design were placed with Harland and Wolff on 29 September with reinstated yard numbers 492 and 493, which were renamed respectively Erebus and Terror on 13 October.

Erebus's turret was originally a spare for the battlecruiser Furious, which was prepared in case the new BL 18 in Mk I naval guns intended for the larger vessel were ineffective.

[15][ii] Learning from the earlier experience with Ney, the turrets were adjusted to increase elevation to 30 degrees, which would add greater firing range.

After a refit completed in August 1939, she was earmarked as guardship at Cape Town, but due to the outbreak of World War II this did not occur.