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.