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.