What is the highest-level spell that can be cast without a spell slot an unlimited number of times?

Question

An 18th-level wizard gains a 2nd-level signature spell. This stipulates that they can "cast those spells at their lowest level without expending a spell slot when you have them prepared".

Are 2nd-level spells the strongest spells that can be cast for free? Or is there an ability, magic item, artifact, or something else that would let you match or surpass this ability (i.e. also cast 2nd- or higher-level spells without spell slots unlimited times)?

Answer 1

Several Eldritch Invocations let you cast spells of 3rd level or higher at will without expending a spell slot.

• Whispers of the Grave requires 9th level and allows you to cast the 3rd level spell Speak With Dead at will.
• Visions of Distant Realms requires 15th level and allows you to cast the 4th level spell Arcane Eye at will.
• Far Scribe requires 5th level and the Pact of the Tome and allows you to cast the 3rd level spell Sending at will targeting a creature with their name written in your Tome.
• Chains of Carceri requires the Pact of the Chain and 15th level and allows you to cast the 5th level spell Hold Monster at will. It does have restrictions that you must target a celestial, fiend, or elemental and must finish a long rest before targeting the same creature again, but as long as you have new celestials, fiends, or elementals to target, you can continue to cast it at will without expending a spell slot.

There are also some magic items that allow you to cast spells higher than 2nd level at will.

• The very rare Ring of Telekinesis lets you cast the 5th level spell Telekinesis at will, but you can only target items that aren't being worn or carried with the spell.
• The very rare or legendary Crystal Ball lets you cast the 5th level spell Scrying at will.

Answer 2

Instant Summons and Forbiddance are Sixth Level

Ritual spells by definition are cast without spell slots (emphasis mine):

Certain spells have a special tag: ritual. Such a spell can be cast following the normal rules for spellcasting, or the spell can be cast as a ritual. The ritual version of a spell takes 10 minutes longer to cast than normal. It also doesn't expend a spell slot, which means the ritual version of a spell can't be cast at a higher level.

Instant Summons and Forbiddance, at 6th level each, are currently the highest level ritual spells.

Either of them may be cast repeatedly, indefinitely. Instant Summons may be cast every 11 minutes by Wizards, Forbiddance every 20 minutes by clerics (plus the time needed to walk approximately 200 feet, since the spell can't overlap with another one that is active), for at least as long as that character can go without resting.

Both spells have as their material component a 1000gp gem. In the case of Instant Summons, the gem is not consumed upon casting, but rather when you actually summon the item. However, each cast requires a different gem, regardless of whether the prior one has been used, so it is possible that the caster will run out of these before any other limit is imposed.

In the case of Forbiddance, as @Adeptus points out, the powdered gem is only consumed when the spell is made permanent, which requires casting it thirty times over thirty days in the same place. If the goal is to cast the spell as many times as possible without stopping, it is likely that the caster's going-without-rest limit will have been reached before the gem powder is consumed.

Answer 3

Several monsters have the ability to cast spells at will. So for example, at level 20 you could true polymorph yourself into a pit fiend and cast fireball at will, it's a 3rd level spell.

Looking you could also true polymorph into an Archaic from Strixhaven to gain at will divination, which is a 4th level spell.

Ki-Rin would allow you to cast major image at 6th level at will as well.

Answer 4

Unlimited 10th level spells: Arcana Cleric + Boon of High Magic + Simulacrum + Wish + Book of Exalted Deeds

This combo allows a wizard to have access to an arbitrary number of 9th level spell slots. This answer by Ladifas outlines the basics of the method. In summary:

1. Cast simulacrum on self
2. Command new simulacrum to use wish to cast simulacrum on the original caster.
3. Repeat step 2.

Since we have the Boon of High Magic, we have two 9th level spell slots, so every simulacrum created would have one remaining after using the first to cast simulacrum. Since the simulacra follow our commands (or can be instructed to do so) we have access to an arbitrary number of 9th level spells at will. It takes some setup, but the up front cost is only a 7th level spells slot and the components for simulacrum.

The above method works for unlimited 9th level spells for any character with access to both wish and simulacrum.
However, an Arcana Domain cleric can use the Book of Exalted Deeds to make those 9th level spells into 10th level spells.

The Arcana Domain cleric can learn wish and simulacrum using their Arcane Mastery feature:

At 17th level, you choose four spells from the wizard spell list, one from each of the following levels: 6th, 7th, 8th, and 9th. You add them to your list of domain spells. Like your other domain spells, they are always prepared and count as cleric spells for you.

Then, when we read the Book of Exalted Deeds, all of our 9th level cleric spells are upcast to 10th level:

Enlightened Magic

Once you’ve read and studied the book, any spell slot you expend to cast a cleric or paladin spell counts as a spell slot of one level higher.

Simulacrum duplicates all of our statistics:

the illusion uses all the statistics of the creature it duplicates, except that it is a construct.

The Enlightened Magic feature is copied when we cast simulacrum.