You attempt to send one creature that you can see within range to another plane of existence. The target must succeed on a Charisma saving throw or be banished.
If the target is native to the plane of existence you’re on, you banish the target to a harmless demiplane. While there, the target is incapacitated. The target remains there until the spell ends, at which point the target reappears in the space it left or in the nearest unoccupied space if that space is occupied.
If the target is native to a different plane of existence than the one you’re on, the target is banished with a faint popping noise, returning to its home plane. 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. Otherwise, the target doesn’t return.
At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, you can target one additional creature for each slot level above 4th.
* - (an item distasteful to the target)
I'm throwing my 2 bits in here as I am looking at limitations and unintended consequences of using this spell on party members.
For my game (so obviously use your own judgement) I have ruled that in planes which mirror or overlap the Prime Material such as the Shadowfell or Ethereal, banishing yourself or a party member would result in the character shifting to the corresponding spot.
This means the caster would need to consider their current location. ie. If they are in a dormant volcano in the Shadowfell this could be a live volcano in the Prime Material and so on.
If used whilst in one of the outer planes, then the location they might be banished to on the Prime Material would be unpredictable as there is no corresponding location.
I have also had to consider exotic races and what might happen to them when banished. That is creatures of dual heritage. I haven't been able to find anything that specifically states a native plane for these so I am considering a percentage chance for them to appear either on the Prime Material or on a plane that represents their second heritage.
As the characters are learning more about the spell I will be trying to provide enough information and caution without giving away all the details, that is something for the players to deduce.
Could be that it all goes well ........ or perhaps the party is split in a very dramatic way..... which will lead to all sorts of follow on adventures and such.