A while ago, my DM let me use Banishment on myself to stop my fall from a likely fatal fall. As I was falling, I readied an action to cast Banishment on myself before I hit the ground. He let Banishment cancel the distance I had fallen, letting me fall the last 10 or so feet safely. But what would end up happening read as written? Would I have just taken the impact from hitting the ground in the Demiplane instead? Or would it actually work in the way he let me use it?
Banishment does not state any relation to momentum.
tl;dr D&D is not a physics engine and the demiplane is "harmless", so no damage from hitting the ground in the demiplane. The results upon returning is up to the DM.
D&D is not a real world physics simulation.
Rules as written do not specify what happens with momentum, because it's not a real world physics simulation. E.g. it's also not a fluid dynamics engine
Going to demiplane does not incur damage.
Spells do what they say they do and banishment indicates a harmless demiplane. Without further clarification in the rules, that adjective indicates that the target does not come to harm as a result of being banished. The character would not take damage from hitting the ground in the demiplane.
If the target is native to the plane of existence you're on, you banish the target to a harmless demiplane.
Returning and falling may still incur falling damage.
The rules do not specify in what manner the target reappears. That much is left to the determination and narration of the DM.
If the spell ends before 1 minute has passed, the target reappears in the space it left or in the nearest unoccupied space if that space is occupied.
The falling rules specify that damage is incurred at the end of the fall. This shakes out one of two ways: banishment ends the fall in the harmless demiplane, or the fall continues during the harmless demiplane.
- If the DM rules that banishment ends the falls in the harmless demiplane, no damage is incurred and the player is not prone.
- If the DM rules that the fall continues during the banishment, then the fall ends after the character reappears and damage is accrued as usual (albeit a minute after it would have otherwise).