Is this combination of Quivering Palm and Haste RAW?

Question

I’ve found a combination that seems rather broken to me, and I can’t find anything stating that the combination itself isn’t RAW, although it definitely seem like it isn’t RAI. However, the combination seems far too OP to be RAW, and I wanted to confirm that this is truly an acceptable use of Haste and Quivering Palm in a combination. There’s a second part to the question, assuming the first is correct.

To make this simple: a level 17 Way of the Open Hand Monk (with Quivering Palm) and a level 17 Sorcerer are in the party. The Sorcerer casts Haste on the Monk. The Monk now has 2 actions (Normal Action + Haste Action). The Monk uses his Haste Action to make one Unarmed Strike against an opponent and spends 3 Ki Points to set up the Quivering Palm vibrations in the enemy. The Monk then uses his regular action to trigger said vibrations and either drop the enemy to 0 HP if it fails the Con save, or do 10d10 damage if it succeeds.

Assuming that the Haste stays up, the Monk may do that 5 turns in a row before he runs out of Ki Points. Thinking of high-level enemies, unless they have crazy good Con saves, even with Legendary Resistances they have a decent rate of failure and this seems like an abuse.

If a 17th-level Open Hand monk has Haste cast on them, can they set up and trigger Quivering Palm in the same turn?

Assuming that the answer to that question is yes:
If that same Monk also had 2 levels in Fighter, could they not also use their bonus action and their Action Surge for an additional Quivering Palm setup and trigger in the same turn?

Answer 1

Yes, a hasted Monk 17 / Fighter 2 could set up and trigger Quivering Palm twice in a round.

Your reasoning all checks out:

1. Use your hasted action to Attack with an unarmed strike and spend three ki points to set up Quivering Palm (the action granted by haste can only be used for a limited set of actions - Attack being one of them).
2. Use your normal action to trigger Quivering Palm.
3. Use your bonus action to make another unarmed strike, spending three ki points to set up a second Quivering Palm effect.
4. Action Surge to trigger the second Quivering Palm.

But is this overpowered?

There's no denying that it's a strong combination. However, activating does have a pretty high opportunity cost - it requires six ki points (about a third of your total), your action surge (one use per short/long rest), another PC to use a spell slot, and you need to successfully roll to hit with both attacks. The game is balanced around a standard adventuring day consisting of six to eight encounters. If your group actually played the game that way then you'd be running on empty fairly soon.

You also need to be 19th level to activate this (full) combination (17th to do a single Quivering Palm in a round). Statistically, very few campaigns actually reach this level of play but if they do it's likely that most of the PCs will have some kind of ability that seems incredibly OP. At level 17 all the party's full casters get level 9 spells - so, as an example, the party's wizard could be doing 40d6 damage to all of the enemies within an area with meteor swarm (admittedly only once per long rest).

So, I'd say this is RAW and it does feel pretty strong, but if your party reaches 17/19th level you'll probably find that your multiclassed Monk isn't the only PC that can do some pretty gnarly stuff.

Answer 2

Yes

It is RAW. The important part here is:

The Monk then uses his regular action to trigger said vibrations and either drop the enemy to 0 HP if it fails the Con save, or do 10d10 damage if it succeeds.

Emphasis mine

Although it doesn't seem like it, arguably, it is balanced. Most monsters at that level would make the CON save, and 10d10 isn't as much damage as a wizard of the same level could output. This doesn't mean it's very effective against lower level or lower CON monsters.

And as Zoma mentioned in a comment on the question, a monster doesn't just stand there and take it, especially if it is a boss. It will learn to shift its body in such a way to prevent the vibrations, or some other in world reason it won't work that often. Although this is not recommended

And of course, you can always talk to your players, and discuss it. If it ruins your encounters, it will end up with everyone having less fun.