I'm running a homebrew where a big plot point is that these massive holes into other planes keep opening and leaking their essence into the material plane.  There's a faction dedicated to dealing with them, but I need to craft a spell that closes the rifts.

The idea was that the spell functions in two ways;

1) When used in tandem with another caster who is on the opposite side of one of these rifts, they have to maintain concentration for three rounds to initiate the rift's collapse.

2) A separate effect when used alone.

The problem is that I'm having trouble finding a good way for the spell to be expressed outside of the purpose for which it was made.  I feel it would make sense if the spell by itself could close arcane gates and whatnot, but that feels a little niche.  Any ideas?

Options:

1) make a custom spell that has a single usage in closing gates - in fact I think this already exists in the planescape book.
Pro: does exactly what you want.
Con: becomes a spell-tax on your party's caster such that they have to take this spell and save spell slots just for the purpose of closing gates.

2) allow another spell (e.g. Dispel Magic) to also function to close gates.
Pro: doesn't take away from spells known/prepared for your casters.
Con: still requires casters to save spell slots on the off chance they need to close a gate.

3) create a magic item that closes gates.
Pro: no cost to casters.
Con: means anyone can close gates so doing so feels less special.

Quote from Agilemind >>

Options:

1) make a custom spell that has a single usage in closing gates - in fact I think this already exists in the planescape book.
Pro: does exactly what you want.
Con: becomes a spell-tax on your party's caster such that they have to take this spell and save spell slots just for the purpose of closing gates.

2) allow another spell (e.g. Dispel Magic) to also function to close gates.
Pro: doesn't take away from spells known/prepared for your casters.
Con: still requires casters to save spell slots on the off chance they need to close a gate.

3) create a magic item that closes gates.
Pro: no cost to casters.
Con: means anyone can close gates so doing so feels less special.

 on #3, what if you made the item only attune-able to casters? Wait...then that makes it an attunement tax. Dang it. Then again, if the OP doesn't plan on giving that many magic items, this might not be a problem. But from what I can tell, this seems like a high magic campaign, so that isn't likely.

Quote from Agilemind >>

Options:

1) make a custom spell that has a single usage in closing gates - in fact I think this already exists in the planescape book.
Pro: does exactly what you want.
Con: becomes a spell-tax on your party's caster such that they have to take this spell and save spell slots just for the purpose of closing gates.

2) allow another spell (e.g. Dispel Magic) to also function to close gates.
Pro: doesn't take away from spells known/prepared for your casters.
Con: still requires casters to save spell slots on the off chance they need to close a gate.

3) create a magic item that closes gates.
Pro: no cost to casters.
Con: means anyone can close gates so doing so feels less special.

 
Option 4) Create a spell that affects creatures and/or objects from other planes in some way and has a secondary effect of closing portals when used on those.
Pros: No tax
Cons: Becomes a “must take” spell.