Erebus and Terror were the two ships commanded by Sir John Franklin on his ill-fated Arctic expedition in search of the Northwest Passage. Both ships became trapped in ice off King William Island in September 1846, and Franklin and his entire crew of 129 men subsequently perished. It was later discovered that some of the men had resorted to cannibalism in a desperate attempt to survive. In 'Geography and Some Explorers', Conrad alludes to this event as ‘the darkest drama perhaps played behind the curtain of Arctic mystery.’
Conrad’s examples are aptly chosen: Erebus, in Greek mythology, was a dark region under the earth through which the dead passed before entering Hades. With frequent allusions to the Aeneid and the Inferno, the idea of katabasis (a descent to the underworld) is an important theme in Heart of Darkness.
David C. Woodman, Unravelling the Franklin Mystery: Inuit Testimony (1992)