I am curious why such dark and morbid names were chosen for these two ships that were lost in the Franklin Expedition. Of course it all seems somewhat prescient somehow in retrospect, but was this part of a naming tradition of the time? I know that the Erebus was the second ship with the name in the fleet, so if it is a tradition, it may date back, but it's hard to find an answer though the usual means because there is so much about the Franklin Expedition itself out there, making searching through key words difficult. Many thanks in advance for any interesting context you can provide!
Both HMS Erebus and HMS Terror were fitting names when first chosen.
The first HMS Erebus, launched in 1807, was built as a fireship (i.e., to be set on fire and sailed towards the enemy fleet), and her hellish name seems quite appropriate for a fireship. She was never used in that role, and officially became a sloop in 1808. She returned to being a fireship in 1809, but was not expended as such, and was converted to a rocket ship in 1814. This was still a good fit for the name, since a salvo of Congreve rockets was a fiery thing indeed. She was one of the ships responsible for the rocket bombardment of "The Star-Spangled Banner.
The Franklin Erebus was the second HMS Erebus, launched in 1826. She was a bomb vessel, with large mortars (for throwing explosive shells, mostly for bombarding fortifications on shore) as her main weapons. Perhaps Erebus was chosen to be re-used as her name since it is appropriate for a bomb ship.
The first HMS Terror, launched in 1696, was also a bomb vessel. Her class-mates were named Serpent (perhaps meaning dragon rather than snake), Thunder, Furnace, Granado ("grenade"), Carcass ("explosive shell"), Comet, Dreadful, Basilisk, and Blast. HMS Terror's name fit with those of her sisters. Four of the next 5 Terrors were also bomb ships, include Terror number 6, the Franklin Terror, of 1813. She bombarded Fort McHenry alongside HMS Erebus - she served alongside one Erebus in war, and her successor in exploration.
Some Royal Navy ships with "dark" names were ordinary enough ships: HMS Cerberus was a frigate, HMS Charybdis a brig-sloop, HMS Meteor a gun-brig (but was followed by a couple of bomb vessels of the same name), HMS Petard a destroyer, HMS Scylla a brig-sloop, HMS Terrible a sixth-rate. Many ships with such names were fireships or bomb vessels. Some examples of RN fireship names include: Lucifer, Rocket, Sulphur, Torch, Vesuvius, Vulcan, Volcano - all suitable fiery companions for HMS Erebus. HMS Lichfield, while bearing a suitably dark name ("field of the dead", or "cemetery"), is more innocently named after the city rather than "cemetery". Bomb vessels beyond the ones named above included: Terrible (not the first HMS Terrible), Thunder, Lightning, Firedrake, Infernal, Mortar, Aetna, and Vesuvius (not the first). In some navies, a third type of ship sometimes deliberately carried "dark names": submarines. One might suspect HMS Sea Devil of being named thus, but it's a more innocent fishy name, "manta ray" (the Finnish WWII submarine Vesihiisi had a devilish name: "water devil").