Does use of a Fireball Necklace count as 'casting a spell', and would it end Invisibility?

Question

I'm playing an Arcane Trickster Rogue and I need to know whether using the Necklace of Fireballs counts as casting a spell in order to work out how it interacts with my Magical Ambush and any use of Invisibility.

Necklace of Fireballs: This necklace has 1d6 + 3 beads hanging from it. You can use an action to detach a bead and throw it up to 60 feet away. When it reaches the end of its trajectory, the bead detonates as a 3rd-level fireball spell (save DC 15).

The Necklace has very specific wording on the description which suggests to me that it creates a spell-effect without the user casting a spell.

Other magical items in our inventory are specific about you casting a spell, e.g.,

Hat of Disguise: While wearing this hat, you can use an action to cast the disguise self spell from it at will. The spell ends if the hat is removed.

Below are the rules for Magical Ambush and Invisibility:

Magical Ambush: Starting at 9th level, if you are hidden from a creature when you cast a spell on it, the creature has disadvantage on any saving throw it makes against the spell this turn.

Invisibility A creature you touch becomes invisible until the spell ends. Anything the target is wearing or carrying is invisible as long as it is on the target's person. The spell ends for a target that attacks or casts a spell.

My reading of RAW is that using the necklace does not count as casting a spell, therefore the target would not be inflicted with disadvantage from Magical Ambush, but if I used it whilst Invisible, that invisibility would be maintained.

I'd appreciate other insights on this, particularly if anyone has any examples of magical items where the 'not spell-casting' is made explicit, so I can compare.

Answer 1

RAW, no and no

In the DMG's section on magic items, there is the main heading "Activating an Item" (p.141). That tells us (emphasis mine)

Activating some magic items requires a user to do something special, such as holding the item and uttering a command word. The description of each item category or individual item details how an item is activated. Certain items use one or more of the following rules for their activation. If an item requires an action to activate, that action isn't a function of the Use an Item action...

The "following rules" refer to the four subsequent subsections, "Command Word", "Consumables", "Spells", and "Charges". It is thus to be expected that using a magic item may require the user to do at least one of of the four 'special' things, but if so we can also expect that the magic item description will tell us which one(s) are required.

The "Spells" subsection says

Some magic items allow the user to cast a spell from the item...Many items, such as potions, bypass the casting of a spell and confer the spell's effects, with their usual duration.

So, we are told twice that only some magic items require casting a spell to use them, and told once that many items do not.

The description of the necklace of fireballs says

You can use an action to detach a bead and throw it up to 60 feet away. When it reaches the end of its trajectory, the bead detonates as a 3rd-level fireball spell (save DC 15).

From this description, we note three things:

1. Nowhere does it say that you need to cast a spell; this is similar to a potion in that a spell effect is achieved without a spell having been cast.
2. It does say that you need to use an action.
3. It says that you must detach a bead to use it (which permanently removes it from the necklace). Thus, of the four possible 'special' somethings, the beads are being activated as consumables.

Some items are used up when they are activated. A potion or an elixir must be swallowed, or an oil applied to the body. The writing vanishes from a scroll when it is read. Once used, a consumable item loses its magic.

It is clear that the bead is activated by being "used up", and not by having you cast a spell. Assuming that you must cast a spell when the necklace description does not make reference to you doing so, makes no more sense then assuming the necklace has a command word when none is mentioned, either.

This answers Magical Ambush; you are not casting a spell, so you do not get to use Magical Ambush.

Whether or not using the necklace ends your invisibility is a little more complicated. Clearly you are not casting a spell - but are you making an attack? Unfortunately, there is no explicit definition in the rules of what making an attack means. We are told that if you are making an attack roll, then you are making an attack - but that doesn't tell us everything that might constitute an attack, for example, Grapple and Shove. The current understanding is that an "attack" is anything that requires an attack roll or which explicitly says it is an attack. Throwing a bead, or casting a fireball for that matter, are neither of these, and thus are not attacks. Your invisibility would be maintained.

Answer 2

It does not.

Using a magic item, even if it replicates the effect of a spell, is not casting a spell.

As you note, some items do count as casting a spell and will explicitly use that term to indicate it. As an example for more direct comparison, let's look at the Wand of Fireballs:

[...] While holding it, you can use an action to expend 1 or more of its charges to cast the fireball spell (save DC 15) from it.

The "Activating an Item" topic on DMG p.141 has a subsection, "Spells" that discusses this directly:

Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise. [...] Many items, such as potions, bypass the casting of a spell and confer the spell's effects, with their usual duration.