How much damage should a Rogue lvl5/Monk lvl6 be able to do with unarmed strike in 5e?

Question

One of the PC's in my campaign is playing a 5th lvl rogue/6th lvl monk. He uses unarmed strikes but has two magic items to boost his damage a bit: a set of bracers that add 1d6 slashing and a ring that adds 1d4 green fire damage.

In last night's session, he did 84 pts of damage against an abominable yeti. His attack action (attack + extra attack) netted him 16 points of bludgeoning damage + 10 points slashing + fire from magic items. He followed with a sneak attack for 19 damage (This is my first campaign to run. I hadn't played D&D since AD&D. I let him use sneak attack with unarmed strike when we started without knowing better and it seems unfair to take it away now that he's getting up in levels) - plus 4 points magic damage. He then burned a ki point for flurry of blows in his bonus action for 2 attacks dealing 9 points (incl. magic damage) in the first hit and a critical hit on the second one dealt 26 damage total.

That's a grand total of 84 points for the monk in one round.

Did I mention he's a ghostwise halfling? A few session ago he beat an adult copper dragon to death with his bare hands. By himself. (Okay, the dragon was trapped in a cave where he couldn't fly and I wasn't rolling the best that night and the halfling made his save against the dragon's breath weapons and frightful presence, but still!).

Aside from allowing the sneak attack, am I doing something wrong here or did he just know how to build a monk that approaches OP?

Only good thing about this is that the ring he wears for the green fire damage is cursed. When he deals out a total of 125 points of green fire damage it's going to explode, dealing him an amount of damage between 50% and 100% of the accumulated damage. It happened once already with a lower damage trigger, but he didn't get it taken care of, he decided to keep using it anyway.

Answer 1

What you describe seems to be normal monk behavior.

In terms of the actions the monk is taking, you are seeing normal behavior. Two attacks with an attack action and two attacks with Flurry of Blows is the monk functioning as intended. Typically, an 11th level monk will get these same four attacks, dealing 1d8+(dex or str) on a hit.

The magic items are making the monk seem OP, but it’s the items, not the monk.

These magic items are adding an average of 6 damage on top of every attack that hits. A 20 dexterity, 11th level monk has an expected damage of 9.5 on a hit. Your monk is dealing 13.5 damage on average before sneak attack damage is applied - a 40% improvement before we even consider sneak attack on top of it.

You seem to be on track with doing something about the ring. These questions may have some helpful advice for moving forward with your game: How do I explain to an AD&D player that items that increase Armor Class in D&D 5e are overpowered? and I gave a too powerful magic item at too low level for a bad reason, what to do?

Either way, you need to talk to your player and decide together if this is presenting an issue for the two of you, as well as the rest of the table. If so, there is some good advice in the two Q&As linked above.

Assuming we allow sneak attack, here is what four hits looks like with these items.

1. 1d6 (unarmed) + 1d6 (bracers) +1d4 (ring) +3d6 (sneak attack) + 4 (ability) = 1d4+5d6+4 = 24 average, 38 max.

2-4. 1d6 (unarmed) + 1d6 (bracers) +1d4 (ring) + 4 (ability) = 1d4+2d6+4 = 13.5 average, 20 max.

This adds up to 64.5 average and 98 max. Sneak attack only contributed an average of 10.5 to our damage here, whereas the magic items accounted for an average of 24 of our damage. So 84 damage including 1 crit actually seems well within the realm of what we would expect, needing only a few good rolls to get there from the average.

For comparison, 4 hits from an 11th level monk without damage boosting magic items has an expected damage output of 38 damage, so your monk, calculating with their items, has an average damage output 70% higher than a pure monk build without these items.

Answer 2

As mentioned in a comment for the other answer; while it is indeed true that you shouldn't have permitted sneak attack on unarmed strikes, there are lots of Finesse weapons that are compatible as Monk Weapons, including (if you use Tasha's Cauldron of Everything at your table) Rapiers (proficiency obtained from their Rogue levels), which means that if they wanted to min-max their damage legally, they could go even higher without even having to change anything about their character. Even without Tasha's Cauldron of Everything, they could legally do the same damage as they're doing in your campaign by just using Shortswords or Daggers; either of which are valid Monk Weapons, and can trigger Sneak Attack, and in the daggers' case, they'd simply use their Monk Die as their Weapon Die because that's how Monks work.

Which is kind of a long way of saying that your player hasn't done anything remarkable or special to make their character powerful. The magic items they've acquired are the culprit.

As a case study, I've run some numbers on how your player's build compares to other possible builds at their level, using their average Damage per Round (DPR) as a number to compare with. This shows us a pretty clear picture:

	AC0	AC11	AC13	AC15	AC18	AC20	AC25
Regular Rogue5/Monk6 (Unarmed or Shortswords)	44.053	44.053	40.712	37.356	32.184	28.528	17.813
Rogue5/Monk6 with Magic Items (This is your Player)	68.053	68.053	62.312	56.556	47.784	41.728	25.013
Rogue5/Monk6 with Rapier	46.053	46.053	42.512	38.956	33.484	29.628	18.413
Rogue5/Monk6 Rapier with Magic Items	70.053	70.053	64.112	58.156	49.084	42.828	25.613

Against a typical enemy a level 11 PC might face (in between the AC15 and AC18 ranges), they're dealing 48%-51% more damage, owing exclusively to the magic items you've given them.

Moreover, the damage you cited in a specific instance of a round where he got lucky? My stats show that in their current build against a creature with AC 15 (which is what an Abominable Yeti has), they're expected to deal at least 84 damage about 3.5% of the time. That's only marginally less than the odds of an average character getting a critical hit on a single attack.

Damage	Individual Odds	Cumulative Odds
...	Represents the odds of this specific outcome	Represents the odds of this outcome or better
+82	0.5733%	4.6219%
+83	0.5007%	4.0486%
+84	0.4385%	3.5479%
+85	0.3852%	3.1095%
+86	0.3394%	2.7243%
...

I think the point has been made. Those magic items are extremely powerful. Now, personally, as DM, I don't mind giving players powerful magic items. The fighter in my game has a Hand Crossbow modded to deal 1d9+1d4 damage per hit, and he's combined it with the Crossbow Expert Feat and Sharpshooter feat to deal around 70 damage on average per round as a level 9 character. I'm fine with that because my campaign plays at a relatively high power level, so having a character that can put out a lot of damage very quickly is fine.

But it sounds like you, as DM, weren't expecting to be running such a high power campaign, and as such, you need to be more careful giving out items like that. Especially damage bonuses: in 5e, damage bonuses on weapons are significantly more powerful than attack bonuses, due to damage bonuses in general being much lower, and thus more potent when acquired. An additional +6 damage per hit (1d4+1d6) might not seem like a lot, but as shown, that's represented an entire 50% boost to overall damage.