In older D&D lore, most teleportation type spells worked like a short and (almost?) instantaneous transit through the Ethereal Plane. I'm working on a monster species with Succubus ancestry, being able to cast Misty Step. So, does it work via the Ethereal Plane, or is it something entirely different, maybe depending on setting and its cosmology?
Misty Step does not transit the caster through the ethereal plane
Traditionally, it's a Fey spell that transforms you into a 'silver mist'
Misty Step was first introduced as part of 4th Edition's Warlock Update as a pact boon for the Fey Pact Warlock. The description given to Misty Step in that supplement was simply:
You instantly transform into silver mist that streams a short distance and reforms, allowing you to flee or maneuver to set up a deadly attack. When an enemy under your Warlock’s Curse is reduced to 0 hit points or fewer, you can immediately teleport 3 squares as a free action.
In the 5th edition spell description, the caster is merely 'briefly surrounded by a silvery mist' before teleporting. This edit to the spell's description is probably related to changes between systems - 5e's spell descriptions are entirely mechanical, whereas 4e made more of a distinction between flavour and mechanical effect (see this answer). In the 5e paradigm, it's important for the spell's description to clarify that you could, for instance, teleport into an airtight, transparent container which would be impenetrable by 'silvery mist'.
4e's Fey Pact warlock was a character who had
forged a bargain with ancient, amoral powers of the Feywild.
Misty Step's Fey origins are hinted at by a couple of 5e features:
- The 'Eladrin' Elf subrace in the Dungeon Master's Guide can cast Misty Step once per day (without using a spell slot) using a racial trait called 'Fey Step'
- The 'Fey Teleportation' racial feat for High Elves allows the casting of Misty Step once per day without using a spell slot.