So I have a player that wants to play a Monk, but have a subclass that is more focus on being an Assassin. We originally designed the character using the subclass Way of the Shadow. However, they player wasn't really enjoying the play style as he didn't really want to use any of the 3rd level features as he isn't big on spells. So I started looking around for alternative subclasses, but we weren't ever to find one that fit how he wanted to play. I thought about, just adjusting the Rogue's Assassin or Inquisitive subclasses when I came across a "Ninja" themed subclass on reddit. After reading through the subclass I actually really like the theme and feel like its more how my player wants to play.

Here is the reddit post and here is a link to the currently updated version of that subclass as of 2022-08-24 @ 12:00 EST on homebrewery. Now I've adjusted somethings from that class, because I do agree with some of the comments that the level 3 features seem quite front loaded. I am still working out the language but you can find it on homebrewery.

The things I changed were his Original Ghost Step to be more like Shadow Step, but at the cost of 1 ki point per 30 feet. The next thing I changed was, instead of giving two tool proficiencies, you instead get to pick one out of three, I dropped Supernatural Movement from the Level 6 gained features (though this might make this level a little to weak), lastly I changed Ghost Strike so that if you used Ghost Step to make an attack it would automatically apply the extra damage from the Sudden Strike. 

Now the Sudden Strike is by default it has its own damage based on your level. Damage at Level 3 is 2d6, Level 6 is 4d6, Level 11 is 6d6 and Level 17 is 8d6. Here is how Sudden Strike works in there example: 

Sudden Strike

At 3rd level, you learn to strike with deadly precision. As a bonus action you can spend 1 ki point to gain advantage on the next attack roll you make this turn. If that attack hits it deals an extra 2d6 damage, the amount of the extra damage increases as you gain levels in this class.
The attack must use unarmed strike or a monk weapon.

Now my problem with this is that as sudden strike is written, at the cost of your bonus action, so you wont be able to use Flurry of Blows it gives you advantage on your attack and if that attack hits it deals extra damage 2d6, 4d6, 6d6, or 8d6 based on your level. That seems really strong as Flurry of Blows does not give you advantage and you have to hit with each attack. For example, a Level 3 monk using Flurry of Blows has to roll 3 attack rolls and only deals  1d4 for each attack assuming they are making an unarmed strike. meaning they are going to deal between 0d4-3d4 worth of damage. However as sudden strike is written in this example, it would deal between 1d4+2d6 or 0d4 worth of damage and only require one attack roll at advantage.

So I have adjusted Sudden Strike to instead read as follows:

Sudden Strike

Starting at 3rd level, you learn to strike with deadly precision. As a bonus action you can spend 1 ki point to gain advantage on your next attack roll you make this turn. If that attack hits it deals extra damage equal to your Martial Arts Dice.

Basically I removed the extra damage table in favor of using the Monks Martial Arts Die as extra damage. I feel this feels more balanced as you are essentially getting advantage on the attack roll and instead of having to roll for the extra damage it is auto applied. So unlike using Flurry of Blows where you have to make 3 separate attacks dealing martial arts damage for a max total of 3d4 damage. In this example Sudden Strike grants Advantage on the attack and if it hits, automatically deals the extra Martial Arts Damage.

So I really like this subclass and I feel like my changes make it a little less over the top on the front end but I'd like feedback on both the original class and my variations as my player does seem far more interested in this subclass vs Way of the Shadow and I like to make sure that I am not missing anything. 

I guess I would need two things from the player: what they want when they say "assassin," and why they want to be a monk.

"Assassin", like "ninja", means different things to different people. Some are more concerned with infiltration and stealth while to others, it just means "I want to do a lot of damage in one hit." It sounds like the player is the latter, which leads to the question, why monk?

This subclass seems largely like it's just twisting monk around to be a rogue. Sudden Strike is just Sneak Attack. What is monk giving the player here that rogue can't? When I see a subclass that's basically "you get this other class's main feature," that's a red flag for me. Build off that class if you want its main feature. Just because there's an assassin rogue doesn't mean you can't make your own.

And ghost step is neat but it's too much for a level 3 feature. In comparison, full spellcasters get Invisibility at the same level but need a full action to cast it and concentration to maintain it and can only do it twice a day.