i would debate if assassin is the best subclass for your rogue to be a ninja. Sure assassinate seems plausible, but the bread and butter of a ninja was their covered operations, gathering intel. Not that much assassinating.
Thus i would propose either of these two as alternatives:
Mastermind. It gives you tools and languages and the ability to mimic speech. While also having a long range bonus action help to aid your allies, ie. distracting ninja techniques.
Scout. Nature and Survival expertise to find your targets, it plus nature to find poisonous plants for use. And the reaction to move half of your speed, as a monk, makes you hard to pin down, playing into the elusive factor of ninjas.
Building on this, I would suggest picking up two levels of Warlock to get Mask of Many Faces. Having Disguise Self available to cast at will fits in with ninjas' aptitude for hiding in plain sight.
Bleh, dipping that wide is really not great. I'd stick with a Disguise Kit rather than try and fit 13 CHA and two fairly dead levels into a Monk build.
Personally, I would start as a Monk, pick up only 1 level of Rogue, and go Monk the rest of the way. If you really really want a Rogue sub-class, then I would take 4 levels of Rogue to get the ASI, going just 3 and stopping is a waste and puts you behind everyone else in the group. With just 1 level of Rogue, you'd only be one level behind on ASI's which is doable.
Keep in mind, that any levels put into Rogue, delay the advancement of your martial arts damage - a major feature of the monk class. Use a Nick/finesse martial arts - compatible monk weapon would allow you to add sneak attack, and get an extra attack each round.
As for the Assassin sub-class, I know its popular, but I would argue that it isn't worth it. It's primary features work on only 1 round of each fight (and only if you win initiative of the target you'll be attacking) - not to mention the crit on surprise only works if your DM actually allows you to surprise enemies - better off choosing something that can be used on every round of a fight. (and you can call anything an assassin)
Although it isn't flavored as a ninja, I'm in a campaign now where one of the players has an 8/1 Monk/Rogue and quite frankly, it melts enemies. 5 attacks per round with daggers and flurry of blows that for 5d8 + 1d6 sneak + 4xDex modifier (without magic, it goes up a lot more if using weapons that add extra dice of damage thru magic). That's also 5 chances to crit each round. In that character's case (20 Dex) that's an average of 46pts of damage/round before magic weapons factored in.
As for the Assassin, getting advantage on something you beat on initiative is no different than simply using a vex weapon for your first attack to grant advantage to your second - and you can do that every round.
Obviously the choice is yours, but I would recommend only 1 level of Rogue and starting as Monk, or start as Monk and take 4 levels of rogue. just my 2.4 cp worth.
Building on this, I would suggest picking up two levels of Warlock to get Mask of Many Faces. Having Disguise Self available to cast at will fits in with ninjas' aptitude for hiding in plain sight.
Bleh, dipping that wide is really not great. I'd stick with a Disguise Kit rather than try and fit 13 CHA and two fairly dead levels into a Monk build.
Personally, I would start as a Monk, pick up only 1 level of Rogue, and go Monk the rest of the way. If you really really want a Rogue sub-class, then I would take 4 levels of Rogue to get the ASI, going just 3 and stopping is a waste and puts you behind everyone else in the group. With just 1 level of Rogue, you'd only be one level behind on ASI's which is doable.
Keep in mind, that any levels put into Rogue, delay the advancement of your martial arts damage - a major feature of the monk class. Use a Nick/finesse martial arts - compatible monk weapon would allow you to add sneak attack, and get an extra attack each round.
As for the Assassin sub-class, I know its popular, but I would argue that it isn't worth it. It's primary features work on only 1 round of each fight (and only if you win initiative of the target you'll be attacking) - not to mention the crit on surprise only works if your DM actually allows you to surprise enemies - better off choosing something that can be used on every round of a fight. (and you can call anything an assassin)
Although it isn't flavored as a ninja, I'm in a campaign now where one of the players has an 8/1 Monk/Rogue and quite frankly, it melts enemies. 5 attacks per round with daggers and flurry of blows that for 5d8 + 1d6 sneak + 4xDex modifier (without magic, it goes up a lot more if using weapons that add extra dice of damage thru magic). That's also 5 chances to crit each round. In that character's case (20 Dex) that's an average of 46pts of damage/round before magic weapons factored in.
As for the Assassin, getting advantage on something you beat on initiative is no different than simply using a vex weapon for your first attack to grant advantage to your second - and you can do that every round.
Obviously the choice is yours, but I would recommend only 1 level of Rogue and starting as Monk, or start as Monk and take 4 levels of rogue. just my 2.4 cp worth.
Playing D&D since 1982
Have played every version of the game since Basic (original Red Box Set), except that abomination sometimes called 4e.