Is there any way to follow a banished creature to the demiplane?

Question

The spell banishment allows you to banish a creature to a harmless demiplane, where they remain incapacitated for the spell's duration:

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.

The idea here is that I want to be able to send myself or an ally after the banished creature to inflict further harm upon them while they are incapacitated.

I considered a couple obvious planar travel options, but came up empty handed. The gate spell seemed a bit too ambiguous to be a reliable method:

You conjure a portal linking an unoccupied space you can see within range to a precise location on a different plane of existence.

I think the challenge here is that you don't really know where the creature is, just that it is on some demiplane, so you can't really pinpoint it as a "precise location". Plane shift requires a fork tuned to the destination plane, and a teleportation circle if you want a precise destination within that plane.

But perhaps there is a more subtle and clever wombo-combo of spells and magic items I am missing. Is there any way to follow a banished creature to the demiplane destination of banishment?

Answer 1

A costly & high level solution: combine Instant Summon and Gate.

The spell Instant Summon (Basic Rules, or the Drawmij version in PB, page 235) allows to mark an item and connect it with a sapphire in your possession. If such item is held or carried by another creature, once the sapphire is crushed the caster knows roughly the position of the creature:

You touch an object weighing 10 pounds or less whose longest dimension is 6 feet or less. The spell leaves an invisible mark on its surface and invisibly inscribes the name of the item on the sapphire you use as the material component. Each time you cast this spell, you must use a different sapphire.

At any time thereafter, you can use your action to speak the item's name and crush the sapphire. The item instantly appears in your hand regardless of physical or planar distances, and the spell ends.

If another creature is holding or carrying the item, crushing the sapphire doesn't transport the item to you, but instead you learn who the creature possessing the object is and roughly where that creature is located at that moment.

Instant Summon does not seem to have any constraint about the item being on a different plane or demiplane: crushing the sapphire is sufficient to know where the creature is.

The strategy

The steps to be taken could be the following:

1. Before the fight, you cast Instant Summon on a small object of your choice.
2. During the fight, with DM's approval and/or ruling, you manage to put the object on the target of Banishment.
3. You or one of your allies cast Banishment .
4. You use your action to crush the sapphire: now you know where the creature is.
5. You or one of your allies cast Gate (PB, page 245), reaching the demiplane.
6. Finish your job.

Caveats

Anyway, there are some issues to take into account.

• You have to spend 1000 gp in a sapphire: depending on your table and your DM, this can be a(n economic) problem.
• You have to spend high level slots: Instant Summon is a 6th level spell, Gate a 9th one. The former could be actually not a problem, if you can prepare yourself for the battle at least a day before and take a long rest.
• In case the battle is near, you have to be prepared in advanced, since Instant Summon has a casting time of 1 minute.
• You have to check with your DM how you can put the object on the creature: for example you (or someone else) can try to stab the enemy with the knife/dagger used as target of Instant Summon and try to leave it stuck in the body, hoping that the enemy won't rip it off. Or the party's rogue can try to put it in one of the enemy's pockets, if any.
• If the above issues are solved, you have to ask your DM if the sentence "you learn [...] roughly where that creature is located at that moment" means that Instant Summon provides enough knowledge to target the demiplane with Gate.