Alternatives to rest for restoration of spell slots

Question

Is there a mechanism (established in the rules) for restoring the spell slots of an arcane caster (particularly interested in Wizard) that does not involve rest?

The method must be external to the caster (e.g. a potion or an artifact), and cannot require the caster to have chosen specific skills or class abilities.

As a footnote, I am willing to consider divine intervention, but that is akin to me just making up a DM ruling to get the job done. I need this effect for a story element at the end of a campaign, and would like to keep it legitimate in the context of what is going on.

Answer 1

Possibly

If you are looking for a consumable that restores spells and not an item like the Pearl of Power or Ring of Spell Storage, then you're either looking at:

• A bunch of scrolls, but that is class depenedent, and may limit what your PCs can cast to a set list of scrolls.
• A wand/staff that is destroyed when used, we see a few of these one use items in the index of magical items in the DMG, but none of them quite do what your looking for. You could however, riff off of the Perl of Power and make it more powerful but single use, or make it break after its charges are expended.
• Making another item specific to your realm like a Potion of Arcane Recovery or some uncommon/rare form of faerie food.

But...

Now, those last two option I mention might not fit with your requirement:

As a footnote, I am willing to consider divine intervention, but that is akin to me just making up a DM ruling to get the job done. I need this effect for a story element at the end of a campaign, and would like to keep it legitimate in the context of what is going on.

But creating magical items and consumables is completely within the rules and rights of the DM, as stated in the DMG, where it gives advice about rarity and cost, and building such items (DMG p. 130-143). If you don't want it to seem like Deus Ex Machina, then make sure the elements already exist in the world. Perhaps they see enemies using such potions/food before the event against the party. That way, them receiving them at some point is just par for course, and not some crazy DM hat pull.

I like the idea of a certain faerie food, because you could attach to it a negative side effect for non-fey (maybe elves are immune to effects as well, maybe not) like addiction or a slight buzz that gives them disadvantage on skill checks or the like.

Potion is the smallest departure from the other types of magical items we see in the DMG.

Otherwise, No.

If you are looking for a ready-made magical item, that is consumable, restores spell slots, and is found in the index of magical items in the DMG I'm afraid you are out of options. The few items listed are examples, and classic items that are cannon to D&D world; but they are a small subset of the items that might exist in a campaign. As such, they don't have any consumable currently listed that does what you want.

Answer 2

There are several magical items which provide solutions to your problem. I will give a sub-set of them from the 5E SRD, but you may find even more by taking a quick look through the Dungeon Master's Guide

A Pearl of Power (pg. 232, SRD): While this pearl is on your person, you can use an action to speak its command word and regain one expended spell slot. If the expended slot was of 4th level or higher, the new slot is 3rd level. Once you use the pearl, it canʼt be used again until the next dawn.

This next doesn't really restore spell slots but allows you to cast more spells per day.

A Ring of Spell Storing (pg. 237, SRD): Any creature can cast a spell of 1st through 5th level into the ring by touching the ring as the spell is cast...While wearing this ring, you can cast any spell stored in it.

And lastly:

Rod of Absorption (pg. 239, SRD): ...If you are a spellcaster holding the rod, you can convert energy stored in it into spell slots to cast spells you have prepared or know. You can create spell slots only of a level equal to or lower than your own spell slots, up to a maximum of 5th level. You use the stored levels in place of your slots, but otherwise cast the spell as normal.

There are lots of options, just take a look at the magic items.

Answer 3

The Pearl of Power (DMG 184) does this:

You can use an action to speak this pearl's command word and regain one expended spell slot of up to 3rd level.

Answer 4

There is one other way to create spell slots on the fly. But, it will only work if you allow me to bend the rules you set forth in your question somewhat.

Sorcerers & Flexible Casting

While you are interested in the Wizard class and you are not looking for class-specific skills, since you are looking for a way to restore spell slots all the same, it's worth considering sorcery points for this as well. Flexible Casting allows you to use a bonus action to transform sorcery points into spell points on the fly.

The following are just brainstormed item ideas that you can adapt. As J. A. Streich says in his answer, inventing items is within your rights as a DM. And if you're going to make something up, at the very least, it pays to make it cool and flavorful. Your players will appreciate the new toys.

• Potion of Sorcery: Grants X sorcery points to an arcane caster when drank for Y time. If the caster does not innately have sorcery points, they gain this many and also gain Flexible Casting to convert sorcery points to spell slots (and not the other way around) for as long as they have 2 or more sorcery points, or until Y time elapses, whichever happens first. If the caster already has these points, then they recover this many points instead.

• Ring/Staff of Sorcery: May contain up to X sorcery points, which are accessible to an arcane caster when worn. If the caster does not innately have the feature, they gain Flexible Casting to convert sorcery points to spell slots (and not the other way around). At dawn of each day, it restores 1dY points. If all the points of this ring are used up, the item becomes a mundane ring.

• Scroll of Innate Sorcery: Xth level spell. Range: ?. Duration: ? (concentration?). Choose 1 creature you can see within range. The target of the spell gains the Flexible Casting ability if they do not have it yet, plus one Metamagic option from the Sorcerer's list for the duration of the spell. If they do already have it, they gain an extra Metamagic option for the duration of the spell. When this spell's duration expires, they lose all benefits granted by this casting.

There are possibly many other kinds of items you can create around this idea. If you do go this route, I suggest you go with the consumables first just to see how they will play out, before you give them a permanent item.