In a recent game our monk used a Tentacle Rod to attack three times (per its normal use), then used his Extra Attack to attack three more times, then used his bonus action to deliver an unarmed strike. For a total of 7 attacks every turn.
Is this, as I believe, an incorrect reading of the rules?
My reasoning for attacks 4-6 are that the Tentacle Rod states (it's long because I'm trying to be thorough):
While holding the rod, you can use an action to direct each tentacle to attack a creature you can see within 15 feet of you.
(Emphasis mine.) It doesn't say Attack action, it says an action, and I would say that, since it specifies an Attack action, and the Tentacle Rod is a magic item that isn't a weapon, that the action would be "use an object"
By the same token, the monk rules state:
When you use the Attack action with an unarmed strike or a monk weapon on your turn, you can make one unarmed strike as a bonus action
If the action isn't an Attack action, then the bonus action cannot be used for an unarmed strike.
This would mean that the monk can get either 3 attacks with the Rod, or take his standard attack, Extra Attack, and unarmed strike, which is still 3 attacks.
This is how I read the rules. Also, getting 7 attacks at 5th level seems to break action economy, but that's more of a "sniff test" than a rule I can actually point to.
I found this question from a year and half ago asking basically the same thing, but about Multiattack. However, Multiattack and Extra Attack aren't exactly equivalent, neither are PC actions and monster actions, so I'm inclined to think it doesn't apply.
Thank you.
P.S. I've seen this player make similar mistakes to his detriment, so I'm pretty sure any mistakes here are 100% accidental.
Best Answer
You are right that this does not work with the Attack action
The tentacle rod gives you a use option for your action. This is different from the Attack action, since the item does not specify that this has anything to do with the Attack action.
To see this, you can compare the tentacle rod saying:
and the class feature you cited:
The tentacle rod does not say "when you take the Attack action, you can make three attacks instead of one".
The relevant action for activation of an item is normally Use an Object (PHB, p. 193):
However, in case of a magic item, it is a different action that is not listed in the PHB (see the "Activating a Magic Item" section on DMG p. 141, and this question); p. 36 of the Essentials Kit rulebook calls it the "Use a Magic Item" action.
Without taking the Attack action, you can make neither an Extra Attack nor a bonus-action unarmed strike
Extra Attack only applies to the Attack action, as you already found. The same also applies clearly to the bonus-action attack from the monk's Martial Arts feature. And there is a second reason that the latter does not work: The tentacle rod is not a monk weapon. The item description says:
...so it is definitely a weapon (this was also unofficially confirmed by Jeremy Crawford in a February 2019 tweet). However, it is categorized as a "rod", which in turn is not a simple weapon (rods do not appear on the Weapons table in PHB p. 149, neither does the item description say it is one).
Since the tentacle rod is not a simple weapon and not a shortsword, it is not a monk weapon. Since it is not a simple weapon or martial weapon (no rods on the Weapons table, and the description does not say), you cannot choose it as a kensei weapon (and thereby make it a monk weapon) either with the Way of the Kensei monk's Path of the Kensei feature (XGtE, p. 34).