How does a mizzium apparatus function in regards to spells with a casting time other than an action?

Question

The mizzium apparatus allows an attuned creature to do the following:

While you are wearing the mizzium apparatus, you can use it as an arcane focus. In addition, you can attempt to cast a spell that you do not know or have prepared. The spell you choose must be on your class's spell list and of a level for which you have a spell slot, and you must provide the spell's components.

You expend a spell slot to cast the spell as normal, but before resolving it you must make an Intelligence (Arcana) check. The DC is 10 + twice the level of the spell slot you expend to cast the spell.

On a successful check, you cast the spell as normal, using your spell save DC and spellcasting ability modifier. On a failed check, you cast a different spell from the one you intended. Randomly determine the spell you cast by rolling on the table for the level of the spell slot you expended. If the slot is 6th level or higher, roll on the table for 5th-level spells.

If you try to cast a cantrip you don't know, the DC for the Intelligence (Arcana) check is 10, and on a failed check, there is no effect.

Emphasis mine. All of the spells one can roll for on the tables for a failed check have a casting time of 1 action (or longer—which still uses only your action). However, there is no stipulation that the casting time of the spell you attempt to cast must be 1 action. This leaves it unclear on how one should resolve an attempt to cast a spell with a casting time of 1 bonus action which fails. What if the caster has already used its action for the current turn? What happens if the attempted spell has a casting time of 1 reaction, like shield? Is there any reason why the item would not allow one to attempt to cast such a spell?

Note that I am looking for rules here (if any)—not rulings. An accepted answer should point to a set of rules which leads directly to its conclusions.

Answer 1

The intent appears to be that the random spell just comes into effect, at the cost of whatever the casting time of the chosen spell was.

There is no clear rules answer here. The best we can do is to try to suss out the intent based on a couple observations. I know you asked for rules, not rulings, but sometimes it just doesn't work out that way. The rules aren't perfect and cannot account for every possible interaction. sometimes rules-informed rulings are the best you can do.

1. There is no restriction on the casting time for the spell we attempt to cast.

The item description simply states no casting time restriction, so I should be able to attempt to cast a spell with a casting time of one bonus action, or a longer casting time. However -

2. Spells with a casting time of 1 Reaction are unclear, but probably shouldn't work.

Spell with a casting time of 1 Reaction can only be cast in response to the trigger stated in the spell description. So the apparatus definitely doesn't allow you to just cast shield whenever you want. It is not clear to me that reaction spells even work at all. The rules for reactions state:

Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's.

The Apparatus does not state that it allows you take reactions, but the rules for reactions require that a feature allow you to take a reaction. Unfortunately, I have to go with a ruling here, instead of a clear rules based conclusion. Based on the above, I'm ruling that reaction spells are not eligible, but there is some room to rule otherwise.

3. Not every spell on the random tables has a casting time of 1 Action.

Rolling a 2 on the 4th level table yields conjure minor elementals, which has a casting time of 1 Minute. Now, normally it takes 1 minute to finish casting conjure minor elementals. It is not said that you have "cast" the spell until the minute is up. However, the Apparatus says "you cast a different spell from the one you intended." Based on this, if you fail the ability check, and roll conjure minor elementals, it is cast completely at that moment, rather than the casting beginning at that moment. Unfortunately, this is once again not entirely clear, so this is my ruling based on what is here.

Given this interaction with conjure minor elementals, I would extend it to bonus action spells. You attempt to cast a bonus action spell with your bonus action, you fail and roll on the table, and you get a different spell, still having expended your bonus action. This is the most intuitive ruling that results in not having to weird stuff with the action economy.

Medix2's answer comes to a different conclusion about shield and covers some similar interactions with other magic items that don't really make things much clearer, which just highlights the fact that there is no clear rules answer with this, and an informed ruling is the best you're going to get.

Answer 2

Having a bonus action spell cancelled by the Mizzium Apparatus should remove only your bonus action

First, I will note that one spell the Mizzium Apparatus can cast in conjure minor elementals which has a casting time of 1 minute. Part of the Mizzium Appartus description says (emphasis mine):

[...] On a failed check, you cast a different spell from the one you intended. Randomly determine the spell you cast by rolling on the table for the level of the spell slot you expended. [...]

Compare this to the Staff of the Woodlands which states (emphasis mine):

You can use an action to expend 1 or more of the staff's charges to cast one of the following spells from it [...]

• awaken [...]

The awaken spell ordinarily takes 8 hours to cast, but because Staff of the Woodlands explicitly only requires an action, it changes the casting time of the spell. Comparing the two, we see that the Staff of the Woodlands says you cast one of the spells as an action and the Mizzium Appartus simply says you cast one of the spells, and I would rule that it doesn't use your action at all.

Ordinarily, a spell from a magic item would assume the same casting time as normal:

[...] The spell uses its normal Casting Time, range, and Duration, and the user of the item must concentrate if the spell requires Concentration. [...]

But there is no way to reconcile this with this item in particular. Imagine you tried to cast a spell and it both got turned into a spell that ordinarily take an action to cast. The item says you cast the spell, no questions asked so you'd suddenly be able to cast arbitrarily many spells by constantly failing to cast a particular spell.

Alternatively you could decide that both your bonus action and action are expended when a bonus action spell fails and you cast an action spell, but this is exceedingly punishing and also makes conjure minor elements a particularly odd scenario would you would start casting a 1 minute spell.

As such, I would rule that whatever part of your action-economy you were trying to use, is the one that is used up. Of course, this hinges on the precise meaning of "before resolving [the spell]" but to conclude anything else would allow for arbitrarily many random spells.

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As to whether or not you can cast a spell such as shield, I don't see any reason you shouldn't be able to. The Mizzium Apparatus states:

[...] In addition, you can attempt to cast a spell that you do not know or have prepared. The spell you choose must be on your class's spell list and of a level for which you have a spell slot, and you must provide the spell's components. [...]

There are no further restrictions listed on what kinds of spell you can attempt to cast, so reaction spells would be fair game (assuming their reaction trigger actually comes up in the first place).

This is similar to other magic items that let you cast reaction spells despite not knowing them such as the Rakdos Guild Signet:

This ring, adorned with the symbol of Rakdos, allows you to cast hellish rebuke. [...]