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.
Best Answer
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)
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
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
From this description, we note three things:
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.