This subclass's reason for existing is to make poison damage viable. All guides I know hate it, and that`s fair, because almost every monster resists it. So I made a subclass to counteract it. Sorry it`s so long. So without further ado..

WAY OF PESTILIENCE MONK

THE PESTILIENCE MONKS ARE OFTEN RENOWNED FOR THERE MASTERY OF CHEMICALS AND TOXINS TO SUPPLEMENT THEIR ABILITIES. THEY ARE OFTEN COMPARED TO ALCHEMISTS AND TRANSMUTERS IN TERMS OF SKILL. THESE MONKS ARE DISTINGUISHABLE BY THERE CRUDE GASMASKS AND PALE SKIN.

TOXIN FAMILIARITY

AT 3 LEVEL, YOU GAIN PROFICIENCY WITH A POISONER`S KIT. YOU ALSO GAIN REISTANCE TO POISON DAMAGE AND LEARN THE POISON SPRAY CANTRIP.

TOXIC KI

ALSO AT 3 LEVEL, YOU CAN USE YOUR KI TO HARNESS THE POWER OF TOXIC CHEMICALS TO YOUR BENEFIT.

TOXIC CLOUD

YOU CAN EXPEND 3 KI POINTS TO EXUME A TOXIC CLOUD TO SMOTHER YOUR ENEMIES. YOU CREATE A 10-FOOT CUBE OF TOXIC GAS AROUND YOURSELF, WHICH LASTS FOR 5 TURNS. WHEN A CREATURE ENTERS THE GAS AREA FOR THE FIRST TIME ON A TURN OR STARTS ITS TURN THERE, THAT CREATURE MUST MAKE A CONSTITUTION SAVING THROW. THE CREATURE TAKES 2D6 POISON DAMAGE ON A FAILED SAVE, OR HALF AS MUCH DAMAGE ON A SUCCESSFUL ONE. YOU ARE NOT AFFECTED BY THE GAS. CREATURES ARE AFFECTED EVEN IF THEY HOLD THEIR BREATH OR DON'T NEED TO BREATHE. THE CLOUD COUNTS AS DIFFICULT TERRAIN. THE CLOUD HEAVILY OBSCURES THE AREA, EXCPET FOR YOU. YOU GAIN BLINDSIGHT FOR ALL CREATURES IN THE GAS. WHEN YOU CREATE THE CLOUD, YOU CAN EXPEND AN EXTRA 2 KI POINTS TO INCREASE THE DAMAGE BY 1D6.

POTENT POISONS

WHEN YOU DEAL POISON DAMAGE TO A TARGET, YOU CAN EXPEND 1 KI POINT TO REMOVE A CREATURE’S POISON RESISTANCE OR IMMUNITY FOR THAT DAMAGE.

DISTINCT ANATOMY

AT 6 LEVEL, YOU CAN TURN YOUR FOES POISONS AGAINST YOU. WHEN AN ENEMY DEALS POISON DAMAGE TO YOU THROUGH A MELEE ATTACK OR AN AREA OF EFFECT ATTACK (A POISONED SWORD OR POISON GAS COUNT, BUT NOT A POISONED ARROW), YOU CAN EXPEND 2 KI POINTS TO FORCE THE CREATURE THAT DEALT THE DAMAGE TO TAKE HALF AS MUCH DAMAGE YOU TOOK (RESISTANCE NOT INCLUDED).

TOXIN ADAPTABILITY

AT 11 LEVEL, YOU GAIN THE ABILITY TO NEUTRALISE THE POISON OF OTHER CREATURES. WHEN A FRIENDLY CREATURE WITHIN 10-FT OF YOU TAKES POISON DAMAGE, YOU CAN USE YOUR REACTION TO EXPEND KI POINTS TO NEUTRALISE IT. FOR EACH DIE OF DAMAGE, YOU MUST EXPEND 2 KI POINTS.

TOXIN OVERLOAD

AT 17 LEVEL, YOU CAN OVERLOAD YOUR BODY WITH TOXINS. YOU CAN EXPEND 7 KI POINTS TO GAIN THE FOLLOWING BENEFITS FOR 1 MINUTE:

• YOUR MELEE ATTACKS NOW DEAL AN EXTRA 1D8 POISON DAMAGE.
• YOU AUTOMATICALLY GAIN THE BENEFITS OF POTENT POISONS FOR THE DURATION WITHOUT SPENDING KI POINTS
• TOXIC CLOUD ALSO ACTIVATES FOR THE DURATION
• YOU GAIN AN IMMUNITY TO POISON DAMAGE

Why though? Poison damage sucks because undead and construct very logically are immune to poison damage. Simply writing in rules that "well actually no, that animate sword that has no body or flesh at all, it can now be poisoned because I said so" just doesn't really fit in the reality of the game. Just use acid damage instead of poison and everything is fine.

I get what you're going for trying to salvage the concept of a competent poisoner in 5e, but I agree with Agile that poison as a damage type is kind of defined by the things that resist it. Once you remove that, it's not really behaving like poison so it just feels like something else that you have decided to use the word poison to describe.

Now if a monk were able to tap into other energy flows beyond ki, disrupting the necromantic or arcane energies powering undead and constructs, that's a pretty cool concept (Way of Disruption?). But it would probably best be represented by force or necrotic or something like that.

But that's just like, our opinion. If you wanted to move forward with this, I would remove the ki cost for overcoming resistance entirely, otherwise you're going to have a tough time. If I was charging ki, I would probably do a 1-minute stance that can ignore immunity but maybe you are restricted in what else you can do.

I would also try to fold the poison stuff into the base class more. For many levels, the only non-expensive way you can reliably do poison damage is with Poison Spray. That kind of makes you feel like on your turn you have to choose between being a monk or being your subclass. Integrate poison into martial arts so you can be both at the same time.