How gamebreaking is this magic item that can reduce casting times?

Question

I've been considering the following homebrew magic item to give to my campaign's BBEG. The flavour/intent is that the villain is manipulating the way time works across the multiverse, and I designed the item around that idea. Note that I don't intend for my players to use this, so I'm more concerned about how this would affect the balance of an encounter, rather than abuse that the players might do.

Amulet of Quick Casting
Wondrous item, legendary (requires attunement)

The amulet has 10 charges and regains 1d10 expended charges daily at dawn.
When you cast a spell of 1st level or higher with a casting time longer than 1 Action, you can expend a number of charges equal to the spell's level, assuming the amulet has the necessary charges. If you do, the casting time of the spell becomes 1 Action.

I've gone over the list of spells with a casting time longer than 1 Action, and, while a couple of spells stand out to me as being overpowered if cast in 1 Action (namely, Simulacrum, Tsunami, Imprisonment and the various Conjure spells), the vast majority seem to have, at best, some combat utility, with the rest not being worth wasting charges to cast with a shorter casting time.

Because of this, I'm not too worried about this affecting the balance of any encounters involving my villain, especially as my players will (by the time they actually fight it) be around levels 12-15, and will have access to their own 7th and 8th level spells. Furthermore, in a single encounter, the charges means that the more "dangerous" higher level spells are likely only able to be quickened at most once.

Finally, given the spell-replication ability of Wish, this seems to be a slightly better (can cast 9th level spells as 1 Action) version of a 9th level spell, rather than anything incredibly powerful.

Therefore, for the rarity, and given the players' levels, is this likely to completely shift a campaign-concluding fight, when the BBEG is something akin to a lich or other high CR spellcasters, or is it likely to have little impact? Ignore any ramifications the item might have if it falls into the hands of player characters, this is entirely for "DM use".

Related question, discusses the impact of casting 1 minute spells in 1 round, but is about a homebrew game rule, not a magic item.

Answer 1

I think you're looking at this wrong

First, why an amulet at all?

The flavour/intent is that the villain is manipulating the way time works across the multiverse, and I designed the item around that idea. Note that I don't intend for my players to use this, so I'm more concerned about how this would affect the balance of an encounter, rather than abuse that the players might do.

If the goal isn't to let the players use it, why make it something they can potentially use and abuse? It's just a McGuffin.

Second, reduce the number of spells you need to deal with.

If you limit it to an innate feature of the BBEG, then you need only deal with a very finite list of spells. There are about 75 official spells (as of right now) with a longer than 1 Action casting time, but your BBEG won't have all of them available.

If the BBEG can't cast Animate Dead (normally a minute) then you don't need to worry if a casting time of 1 Action is balanced or not.

Make it a legendary action instead

Give them 3 Legendary Actions per round

Bend Time. As a Legendary Action, BBEG can speed up the preparation time of a spell. Each use of this action builds up a charge of potential spell energy worth three spell-slot levels. This can be stacked with other uses to build up a greater charge. All charges are lost at the end of the BBEG turn. As an Action on their turn, they can unleash this charge to cast a spell that would normal take longer than 1 Action as 1 Action. Using the charge in this way dissipates the complete charge.

So here are the highlights:

• They are still casting their spell as part of their normal action
• They can build up three uses to cast a 9th-level spell or below. Or two uses for a 6th, and one use for a 3rd and below.
• The charges don't last so they can't use it a bunch during a slow round then suddenly whip out a 9th level spell.
• The charges don't last so they can't use it a bunch during a slow round then use a 4th-level one round and a 5th-level the next. At least not without using up a LA.
• You can give the BBEG other LA so they are not always just building up charges
• You can weave into the narrative that the amulet is required for the LA, but since the players have not studied time and space like the BBEG, they do not get the Bend Time ability; it's just a pretty amulet.
• The BBEG only has so many spells available to them at a time so you only really need to consider the ramifications of a few spells instead of six dozen.

Make it easier on yourself

Remove the judging every possible spell and just focus on the encounter.

Answer 2

Tiny Hut alone will make this item overpowered

Tiny Hut is a third-level spell which creates an impenetrable barrier in a 10-foot radius. Any creature, object (including projectiles and reach weapons), or spell effect that was not in the area when it was cast cannot enter or exit the area. Meanwhile, the caster's friends who were inside the area when it was cast can enter or exit at will.

Normally, the 1-minute casting time of this spell means it is useful only for setting up safe locations to rest. Allowing it to be cast in combat would make it, while arguably not game-breaking, better than the 6th-level Globe of Invulnerability while also not requiring concentration.

How to use the Hut in combat

The caster and their allies can then move out of the hut on their turn, perform their actions, and then move back inside. This limits their opponents to only readied actions, which prevents them from taking other reactions and greatly restricts their ability to react. Enemy spellcasters in particular suffer because they can no longer cast a spell in the same round as using a reaction spell like Counterspell (because activating a readied action takes a reaction) nor Concentration spells (because readying a spell takes concentration).

Its 8-hour duration also allows the caster and their party to concentrate on spells without danger, cast other spells with long casting times, such as Fabricate or Glyph of Warding, or even have most of their companions take a Short Rest, while one occasionally sallies out of the Hut every half hour or so to prevent their opponents from doing the same.

Dispel Magic is a viable answer (perhaps the only viable answer), but I would argue that a spell which wins the fight unless dispelled is overpowered at any level.