Monstrosities can temporarily turn into snakes whose size reflects their body at will every 2 hours, as an action(10 Minutes), bonus action(5 minutes), or reaction(2 Minutes) Aka two turns, one turn, and basically what could be a dodge of some sort) (nothing about them changes aside from their appearance)

Elders Tongue: All Yuan-Ti seem to naturally understand Common, Draconic, and Abyssal

And, at level 7 the Monstroctiy can call all Snakes in a 50ft radius and become allied with them for 24 hours. This effect can only be activated once a day whenever the Monstrosity casts Animal Friendship as a cantrip 

Their grotesque mouths contain and excrete poisonous saliva that's acidic to most organic matter. It deals 1d6 poison damage while in its liquid form and 2 poison damage per in turn a 5-foot cloud in its gas form. The Spit also gives them a very potent bite that deals 1d8 poison damage on contact. All 3 build up throughout the sleeping process and each can only be used ounce per short rest. 

All Monstrosities are immune to their poison, which has such a faint scent and smell its undetectable to all who don’t thoroughly check beforehand

A level 5 Monstrosities can use their sharp and elongated tongues to harmlessly infect living tissue with their poison which quickly causes temporary blindness and deafness(One Turn). This can also only be used once per short rest and at level 13 this can be used to instantly take out anything smaller than large as a sneak attack.

Yaun-Ti Monstrosities have thick and slimy scales that give them resistance to all magic, acid, immunity to poison (and being poisoned) at the cost of a weakness to electricity 

Mutative Rot: Monstrosities are considerably weak especially compared to their Malison brethren (-2 to STR, -1 to CON, and disadvantage on both). The effect this has on their face brings an advantage to Intimidation and a disadvantage to Charisma while not covered. As they grow this muscular weakness begins to affect their skin in a similar vein to excessive molting. So at level 7, Monstrosities learn to shape their constantly dying skin into small objects, clothing, and appendages like thin wings that can be used to temporarily glide.