I’m playing a swarm keeper ranger in curse of Strahd. We are about to go to level six, my swarm is ghost themed. We don’t have a monk in the party, what would be a good explanation for multi classing into monk? 

Quick thoughts:

• One of the ghosts is a monk and taught your character some of their techniques.
• Your character finds that taking lives has some effect on the swarm, maybe expanding it if that could be seen as a bad thing or somehow weakening it if it's purely helpful. The character begins studying unarmed combat to limit the issue.
• Flavor your ki points as drawing power from the ghosts in the swarm.

So would a ghost powered monk be a shadow monk, or a mercy monk? Shadow makes more sense connecting to the afterlife, but having the feeling of being between life and death, feels more like a mercy monk. 

Long death monk could be fitting.

Personally I have never played a monk flavored monk. 

As an example I had a ranger monk reborn. He died protecting others and the hour was just right, so the sentinel goddess known as the Twilight Mother brought him back as a reborn. 

He was absolutely miserable for years. He was in constant pain and no longer had to eat or sleep, so days were unbearable and without a purpose. He came across an old monster hunter master, who began to teach him. 

Eventually he learned to find meaning in supernatural mindfulness. He grounded himself in his surroundings through smells and beauty and flavor and touch - so much that he learned to sense the very cosmic energy that surrounds all things. 

To emphasize, I gave him all the sense boost feats and skills I could get, in addition to the revised ranger stuff. I reflavored many things to support the sense thing, like ranger initiative bonus, monk ac, darkvision, alert feat, deflect missiles, ignore diff terrain, tracking etc. All built into this theme. 😄

Point is: He was never any monastery dude. He was a ruthless Punisher/Witcher type of wandering vigilante, who kept himself sane by chewing spices and enjoying beauty to a supernatural extent. 😄

So I think you can just make whatever you find awesome and it will probably fit in the monk design just fine. 

This is actually really intruiging, is Monk Ranger actually viable? And if so how would one build such a character?

Quote from XavisDOS >>

This is actually really intruiging, is Monk Ranger actually viable? And if so how would one build such a character?

My character performed really well both in and out of combat. Keep in mind that it was revised ranger. I think One DnD UA ranger would work too. The one they introduced a while ago with spells at level 1. 

I'll try to keep this brief. You can ask for more details. We had some DM allowances  because it was a 2 PC game. 

Optional class features were all allowed. That is mandatory for this to work. 

Build Foundation (First. You can read the concept in the end)

Started at level 6. (Level up plans described later)

Level 1 ranger (first), level 5 monk

This is a rough build. I won't go into stat detail, because it won't matter for a different stat system anyway. 

Low cha, low int, high dex, high wisdom, medium con (14 for me), str neutral 10. 

Strengths: Wins initiative, advantage on first turn, huge burst damage on first round, decent damage without KI points, takes control of the battlefield, great off-combat utility stuff, works well unarmed and armed, dex and wis saves are strong. Monk gets the longsword.

Weaknesses: Lowish HP, poor int and cha. AC is solid 19 with my stats, but would be lower with standard array. Works best on his own, so not the most fun in a big party (but I think this is a monk problem in general). 

A more detailed build and how it works. I'll try to pick the important aspects for this build. 

Ranger revised 1

Martial weapons. Advantage on initiative. Nice flavor and utility stuff for the monster/witch hunter wanderer concept. Favored enemy works great with monk's many attacks! 

Monk 5

All the normal monk stuff. Dedicated weapon is important. You make the longsword or another versatile weapon your dedicated weapon for that d10 dmg die. 

Alert feat to ensure winning those initiatives. Worked well with the supernatural mindfulness thing too. Dex+5+adv. 

I went shadow monk, so at level 7 I got Shadow Step. The point is to shadow step or sneak up to the enemy for a brutal assault. You get your surprise attack, then you win initiative and unleash all hell by utilizing the ranger's advantage against enemies who haven't acted in the first round. So you really want to win those initiatives. 

If you don't get a surprise attack, you can usually use your monk mobility to get to the enemies either way, when you win initiative. Long death would have worked too, but I wanted a more proactive subclass. 

You use your multi-attack with the d10 longsword. If you spend KI-points on them, for example stunning strike, you get to use your bonus action for another longsword attack. 3 x d10 attacks in one turn. Or you can use flurry of blows, but I think the longsword attack was a great option to spare KI-points if you already used one on your main attacks. 

So this is basically a kensei monk, but actually good and fun. :P 

If this monk gets that first turn, the enemy will be suffering, especially if they are their favored enemy (+2 dmg). 

Possible paths: 

Further into ranger for Hunter's Mark and fighting style. Pairs well with the many attacks. Level 3 would get you +10 speed on your first turn AND another attack on your first turn. 

Fighter for Action Surge to make even more use of that first round advantage and favored enemy. You might want to go all the way to level 3 for Battlemaster, though. Because at this point in your multiclassing you will have lost quite a few KI points, but now you get Superiority dice to make up for those. 

Your Martial Arts die progress will suffer even further, but the Longsword will make up for that quite a bit. 

If you look at a level 20 character, which is usually not worth building around: 3 ranger, 14 monk, 3 fighter. At that point the amount of damage you can deal on your first round is INSANE. But I didn't really expect to go that far, so the build worked well from the start. :) 

You would also lose two ASI:s, but you would get two fighting styles and lots of other perks. This might be a major problem with the standard array, because the Alert feat is so important. So you would only get two actual ASI:s.

But if you get fighter or ranger to 4, you get the ASI. Then I would probably drop the other one to 2, probably ranger. I would want that Diamond soul from Monk. Of course ranger 3/monk 17 would work well, as would ranger 1, fighter 3 and monk 16 or ranger 1/monk 19. Lots of possibilities. 

Basic outline: 

Reborn monster hunter, designed to strike first and strike hard. 73 point point buy, but I don't think this is mandatory. It would probably be a bit less optimized and more versatile with more diverse stats. I gave it a proper zombie look and smell, so cha was 6. Intelligence was also 8, because he didn't remember his past life, so he was very uncivilized. Just hunted monsters in the wilderness. Couldn't read for example. But wisdom was 19, so I played him as the street smart fella. He had sharp wits and he was clever right on the spot, but knew nothing. 

He was a rather kind and helpful man, but violent. His grim aura made people in villages pick fights with him and even try to kill him. So he learned to strike first before they could gang up in him. 

His KI was represented as supernatural mindfulness of cosmic Jedi-like energies that surrounds all things. He kept his undead self sane by focusing on smells, taste, surroundings etc. 

I want to add: This build was very campaign specific, as optional features and revised ranger were allowed. 

But the basic foundation was always the Longsword profi combined with Dedicated Weapon. So that foundation is nice on its own, even if you don't get this First Round Advantage concept. :)