The players aren’t the only ones who have access to this spell, as enemy spellcasters might be able to use it against you. If a player gets banished, then they are sent to a harmless demiplane and are incapacitated while there, returning after ten rounds. On the off chance they are playing an extra planar creature, then they are sent back to their home plane. It’s up to the rest of the party to break the spellcaster’s concentration to get them back! Having enemies with the banishment spell can be a rarity, as there are a lot of other fourth level spells that would better take up the spell slot and can deal actual damage to a party besides simply banishing one member for a minute of combat. Still, if the party does encounter it, then they follow the same rules the monsters would.