Closing a natural rift between planes

[eknock]

In the campaign I'm running at the moment, there are a number of rifts/weak-spots between the material plane and other planes.
There's a group of adventurers in the world who have closed one in the past, though I'm not sure what would be a believable method for accomplishing this.
Any ideas?
The group included a wizard, an eldritch knight, a cleric and a druid, so a magical solution is a possibility

[NecroDancer]

Perhaps the spell plane shift plus an arcane/religion check?

Or maybe the rifts require a sacrifice, the players must offer up their spellcasting ability (or someone else's) in order to seal the rifts.

[mgshamster]

Artifacts can be a good choice. Perhaps am artifact if causing the rifts and they need to find it to close them. Or an artifact is out there that will heal the "natural" world and allow thegnstursl strength of the planet to close the rifts.

Or maybe the cause is on the other side of the rift, and they have to travel through to close it. But this ends up with them being trapped on the other side and now they have to find a way home.

Maybe time flies differently in the other side, and the cause of rifts this time is the heroes from last time trying to find their own way back home (not realizing that its been hundreds of years on this side and only 2-3 years for them).

Or maybe the rifts are natural, and every so often when the planets and stars are aligned, they open up. The heroes just have to wait it out. Meanwhile, nasty critters are coming through and the heroes have to stop them and protect the world until the planets move out of alignment once more and the portals close.

[RumoCrytuf]

Wizard: Uses their arcane might to shut the rifts.
Eldritch knight: Something similar to the wizard, just not as powerful.
Cleric: Invokes the power of their god to close the rifts
Druid: Uses nature magic and whatnot to restore the natural order of the world

Hope this helps! :D

[arrowed]

If you haven't read the Order of the Stick, this setup seems eerily similar to certain aspects of the plot. Assuming you haven't, there are minor spoilers in the link below.

Oots comic #0276 (http://www.giantitp.com/comics/oots0276.html)

The gist of the strip is that the planar rifts could be almost-permanently-sealed by a spell cast by an arcane and a divine spellcaster working together, but both had to be epic-level. Since there are no epic spells in D&D 5e (yet) a custom pair of 9th level spells seems reasonable.

[eknock]

thanks all for the advice, I think that moving forward, closing rifts will be done using any spell a player can justify and a hefty skill check

[Joe the Rat]

Rituals.
Closing a rift should take a bit of oomph, and a bit of time, and a ritual spell seems like a good option.
That means:
1) you have to research the spell / find a copy of the ritual
2) be strong enough a caster (and/or Arcana/Religion check your way into it)
3) have any necessary components. Expensive (reusable or one-per seal) foci, specific ritual accoutrements, additional casters

What they haven't done yet, and would be really good, is have ritual spells that require multiple casters, or gain benefits from having additional casters.

I'd use what arrowed suggests, and have this be an arcane/divine group ritual. The definitions of "arcane" and "divine" are fairly fluid, so you may need to spell out what that means.

So something like
Seal Portal
(whatever you are requiring)th level abjuration (RITUAL ONLY)*
Casting time: Whatever you want to add to 10 minutes
Range: Close enough to be ominous
Components: V, S, M (a crapton of powdered silver & iron (consumed by the ritual), candles, latin chanting choir, 1,000gp gem/artwork to act as a lock, whatever you want) Special: Requires at least two casters to complete.
Duration: Instantaneous (permaseal), Permanent (lock can be "dispelled"), x millenia, or Special

This ritual seals portals or rifts between planes.
[it either works, or each caster needs to make Arcana/Religion (either, or one of each) checks against a DC set by the rift. Additional participants in the ritual can Help with these ability checks]
Creatures trying to pass through the portal while the ritual is being cast must succeed in a Charisma saving throw vs. the highest spell DC of the spellcasters participating in the ritual. (Or leave this off and let the players figure out they need to drop a Protection from Good and Evil on the zone). A creature that fails this roll may not try again for (enough time to not be an annoyance, but short enough you could let them try again before the ritual completes).
(Reversable - you could corrupt this to open rifts: add something fun like a blood sacrifice to the component list)

* - I want to put this as a ritual in part for the "takes time," but also to allow someone with the Ritual Caster feat to participate.

Of course, you could just make up something. Here's the things you need to have, you need to put this many levels of wizard/sorcerer and cleric/druid/ranger/paladin spells into it. bards and/or warlocks are arcane, or can contribute to either side. Or seal with an Intelligence (Arcane/Religion) check with a DC of Absurd, but the DC is reduced by spell levels pumped in, etc.