Mystic Ninja, a Monk with a dash of Warlock/Rogue...?

[Ichtequi]

So my DM has dropped feats for our campaign. I've been thinking of fun cool ways to make a unique assassin with a few fun mechanics.

After a bit of reading through the PHB I thought it might be cool to make a mystic ninja style character that specializes is doing a ridiculous amount of burst damage right at the start of a fight. I want to use flurry of blows and hex together to get a good amount of damage in, with the assassination surprise attacks to get a crit and sneak attack in at the beginning. Darkness with the shadow step class feature seems like it would be great for mobility and stealth...

Of course the downside is that I either have to miss out on Diamond Body or 2 whole Ability Score Improvements. Is there a better way to do this? Maybe going fighter instead of Monk? I don't really mind if it's not the best build in the world, as long as it isn't cripplingly bad. Any thoughts on the best invocations to get? I was thinking Life Drinker and Devil's Sight. If I did go to 4th level on the Rogue or Warlock, which should it be?

[Arelai]

Instead of Warlock and rogue, take 2 levels of revised ranger for hunters mark, favored enemy-humanoids, and dueling fighting style. And the biggest is adv on initiative and advantage on hitting a target that hasn't moved yet.

Then straight monk. You get +2 against humans, +2 on your first hit, advantage for a more likely crit. So in a turn against a humanoid is Advantage: d4to8+4+mod, d4+2+mod, d4+2+mod, +d6 hex. If you're mod is 3 that's almost 30 damage average, 6 less against humans.

[RulesJD]

Honestly, getting bursty damage on a Monk is pretty tough.

I would recommend (for your intended build) getting Monk up to Diamond Soul. It is just too good not to have.

May be a few levels of Rogue for Sneak Attack as you said, but I wouldn't go too far up. Personally I would go Swashbuckler instead for more often guaranteed Sneak Attacks + auto-disengage + increased Initiative from that.

[Specter]

For first round damage, you definitely want Assassin.

I'd suggest warlock for hex, but unless you cast it beforehand you'll waste your bonus action attacks. I'd go Monk 17/Assassin3; Opportunist is a very good ability, since you can also get another reaction crit if one of your buddies attacks the target.

[Yagyujubei]

Ive played a monk 14 / warlock 6 in a campaign that was insanely fun to play. WotS monk obviously and pact of chain/fhey patron for the warlock.

Pact of chains got me access to Quasit which i reskinned as a Yokai that took the form of a crow
Fhey pact got me an AoE fear and misty step
For spells I took Hex, darkness, AoA, Hold person, Mirror Image, Major Image and Gaseous Form
for invocations i took devils sight, mask of many faces and voice of the chain master

I ran around with two daggers as monk weapons and while i didnt do tons of damage (basically 4d8+4d6+20) but I had unstoppable stealth/scounting, incredible mobility and battle control with stunning fist it was seriously ninja as ****

It was a really tough choice between taking a 15th level in monk for timeless body or a 6th level in lock for misty step, but i figured its not like the campaign was gonna last forever so who cares if my character would.