Is there a way to cast a spell that isn't in your spell list as a Warlock?

Question

Magic Jar isn't on the Warlock spell list. Neither is Wish. Is there any way to cast magic jar as a Great Old One warlock without multiclassing? All official rules and items are available. Negotiation with DM is not possible.

Answer 1

As you said items are valid, there is an option. But the existing option is uh....powerful. Enough that if your DM isn't willing to edit your spell list, they'll quite certainly not give you this. And your other options are much the same.

I'm going to provide some information other than "No" anyway because, again, you said "Valid Items" and, well, you can't get magic items if your DM doesn't want you to have them.

Officially existing item

There is exactly 1 way I can find for you to gain access to this spell directly without multiclassing or talking to your DM about creating something new. You are going to need to acquire the Artifact-tier magic item Demonomicon of Iggwilv from Tasha's Cauldron of Everything.

Among its many, many powers is...

The book has 8 charges. It regains 1d8 expended charges daily at dawn. While holding it, you can use an action to [...] expend 1 or more of its charges to cast one of the following spells [...] magic jar (3 charges)

Unfortunately, acquiring an artifact-tier magic item will still require the cooperation of your DM.

Gaining Access to Wish

This is the eternal answer to any "Can I cast [X]" question. Here are a few ways to do it.

• Acquire a Ring of Three Wishes or a Luck Blade
• Acquire control of a Genie that can cast Wish (such as one in an Iron Flask)

And that's it...

Apart from these, you're settling into "Please homebrew me a Wand of Magic Jar" territory, which doesn't seem much different than talking to your DM about changing your spell list. Though, for reference, in accordance with the rules on Magic Item Power by Rarity a 6th level spell like Magic Jar could be attached to a magic item of Rare rarity. So, in theory, you could have a Wand of Magic Jar (Rare).

Answer 2

The Genie patron

Warlocks with the Genie patron (from Tasha's Cauldron of Everything, p. 73) have an expanded spell list that is adding wish as a 9th spell level option to your available spells (accessible through the Mystic Arcanum feature).

Wish also of course will allow you to cast magic jar.

Since you updated your question to specify you are looking for a solution for a Great Old One patron warlock, Tasha also contains optional rules to switch your subclass at levels 6, 10 or 14. While you could not cast them as a Great Old One warlock, you could change your patron to cast them, without multiclassing. These rules are entirely by the book, but require your DM's consent.

And my understanding is that you do want to be able to cast the spell as a warlock, not via shenanignans that make you something else than a warlock. With no collaboration by your DM to make this happen, you are out of luck. All the various magic items to make this happen are likewise gated by the DM.

Answer 3

Not without negotiating with the DM

Your character does not have access to the spell based on his character class. All means to gain access to it through magical items mean, you need to convince the DM of one of the following things:

• Either that you can have the items, such as those mentioned by guildsbounty
• Or that you are allowed to swap your Patron to the Genie as that grants access to Wish (via Mythic Arcanum) and thus indirectly Magic Jar as described by a certain Hobgoblin, and the changing subclass rules can be found in Tasha's Cauldron of Everything: that is an option at 6th, 10th and 14th level.
• Or that you can buy a scroll (not spell scroll) that has the same effect as Magic Jar.

In all cases, the DM is the final arbiter because they decide if an item is in store, or if you may change your character beyond what the rules say you can do. Even with changing the subclass according to the rules in Tasha's, the DM is still allowed to veto your choice for he is empowered to do so by the book - and that subclass changing rule is optional and thus requires DM approval to be in play in the first place.

As a result, you need to negotiate with the DM for all ways to get the spell you want, even if you do sacrifice something for it.

Talk to your DM

You might say you can't negotiate with them, but you can not know that until you try! Ask them nicely if you can acquire one of the points, and if they deny it, you have to live with that answer. But you also might be positively surprised, as many DMs are open to discussion if you just stay civil and friendly in your inquiry and don't try to force your issue.

When talking to your DM, make sure that you have arguments for why you want the specific spell, and ideas on how it impacts balance to gain it, as addressing those can sometimes help make a DM more amenable to any changes: if you show that you can be responsible and are willing to compromise, so might the DM.

From personal experience as a GM and player in many systems, it really helps to have an open mind in such discussions - I have allowed some quite silly stuff, and given in to players wanting stuff that was on paper imbalanced, as long as they gave me good reason or self-restraint to how they wanted to use their new toy. But do note that if players abuse the new toys, a good GM is not above taking the toy away again.

Answer 4

If your DM is willing, there is some wiggle room in the rules for spell scrolls. The rules in the DMG state:

Unless a scroll’s description says otherwise, any creature that can understand a written language can read the script on a scroll and attempt to activate it.

However, there is a contradictory section in the PHB under Magic Item Rules that says:

A spell scroll bears the words of a single spell, written in a mystical cipher. If the spell is on your class’s spell list, you can read the scroll and cast its spell without providing any material components. Otherwise, the scroll is unintelligible.

Depending on how strict your DM is and how they interpret those rules, they may or may not let you cast a spell from outside your list if you can get your hands on the right scroll.

Personally, I would allow it in my games. As a DM I have control over when (or if) you can locate the scroll and how much it costs. On top of that, even if you do manage to obtain the desired scroll you still have to make an arcana check to see if you can cast the spell without side effects. If all of that works you still have to hope the attack roll or saving throw is in your favor. That's more than enough guardrails to prevent abuse in my book, but you and your DM will have to decide for yourselves.