Well, if you don't mind making a pact with some powerful entity, you could get 2 levels of warlock. This will give you access to a few spells, some of which perhaps being interesting (both stats and flavor-wise) for your character. But most of all, this will give you access to 2 invocations.
One of these invocations should be Devil's Sight, which grants you the ability to see through any form of darkness, magical or otherwise, for up to 120 feet. This is more than enough to cover the 15 feet of the darkness spell. Cast it on your clothes, and bring terror to your enemies.
While this is a good strategy (generally favored by blade pact warlocks), it has some limitations. While under the effect of the darkness spell, your allies cannot see you. As such, any spell requiring line of sight that they would like to cast on you simply cannot work. Healing you might be problematic, as it requires your allies to be able to touch you. They might know that you are smack in the middle of that sphere of pure black darkness but, once they get inside, it's suddenly not so easy to find you anymore.
These limitations also affect your enemies, however. Any attack from the outside of the sphere should have disadvantage as your enemies, even if they can approximate your position, can't exactly see you to aim properly. Enemies inside the darkness should also suffer from disadvantage when attacking you, unless they possess some form of blindsight or a similar ability. When you attack them, you should have advantage on the roll since you are functionally invisible for them.
The biggest cost of this strategy is the fact that it requires multi-classing. While the lvl 19 and 20 abilities for monk would not really be missed (realistically, few campaigns will reach these levels), it will still set you back 2 levels in obtaining you other core monk powers. Note that doing this would get you 2 invocations, however, and that some of them can be quite interesting for a ninja-esque character:
Armor of Shadows (cast mage armor at will, which might be better than
using your wisdom for AC if its under 16),
Eldritch Sight (cast detect magic at will),
- Eyes of the Runekeeper (allows you to read anything, even languages
you don't know... useful for spying!),
- Gaze of Two Minds (share the eyes of a willing target, more spying!),
- Mask of Many Faces (disguise self at will!), etc...
Sacrifice 2 monk levels and multiclass as a warlock. Gain some minor but potentially useful spellcasting, as well as the ability to see through magical darkness and one more ability of your choice!
Our party barbarian wanted to choke out another NPC half-orc that she had challenged to a fight. Our DM followed the following pattern:
- On the strangler's turn, a regular grapple contest is made, with the strangler at disadvantage. This represents the difficulty in getting your hands/forearms/legs around the neck of the enemy. Since it's simple disadvantage, you might be able to negate it if you have the enemy prone or you are enraged (as was the case with the barbarian).
- On the target's turn, a strength (athletics) or dexterity (acrobatics) contest is made. Now the target is trying to break the chokehold before they risk suffocating. If they break the hold, the grappling ends. If not, continue to step 3 immediately. They can choose to try something else besides making the contest, but if it doesn't break the grapple somehow, proceed to step 3.
- The target makes a CON save versus 8 + proficiency + str mod of the grappler. If they succeed, they do not pass out, and you start over at step 2. If they fail, they fall unconscious until the end of their next turn. The save DC was a pretty arbitrary calculation chosen to match the save DC calculation for other spells and abilities.
The rationale for the short time to for the orc to become unconscious is that a proper chokehold in real life will cut off your blood supply to the brain and render you unconscious very quickly, and we also didn't care to sit there and roll repeatedly. One piece that is missing here is making such an action fatal, but I suspect that is where you would simply apply the suffocating condition to keep things simple, until the grapple is broken or target dies.
In our situation, the party barbarian choked the orc unconscious and then released him. We were in a drow prison camp, and a female drow showed up and instructed that the barbarian drag him off and feed him to this giant spider. So we didn't have to resolve the fatality ourselves.
We applied the same rules much later to a gnome we were capturing. I suspect that if the target was not a humanoid, different rules would be desired - past a certain point, attack by strangulation does not make sense and should probably be dismissed out of hand or reduced to an improvised weapon attack. It's also pretty limited in a big fight, since enemies can take the help action to give advantage to the grappler on their side, or grapple the grappler and drag them away.
The rules for holding your breath are on page 183 of the PHB.
The easiest way to extend this is to use a Bottled Breath, from Princes of the Apocalypse. This will let you hold your breath for an hour. Using Polymorph to turn into a Plesiosaurus or a Giant Octopus will likewise let you hold your breath for an hour. If you're really serious about this, though, you should turn into an air genasi. This will let you hold your breath literally forever.