Pre-requisites:

Yuan-Ti Pureblood race and a ritual that involves drinking a potion of rare herbs. 

By performing rituals a Yuan-Ti Pureblood may further transform towards a more snakelike version of Yuan-Ti. In doing so the following abilities are gained as the Yuan-Ti develops venom sacs and fangs. If the Yuan-Ti accentuates these features they find that they are more acceptable to Yuan-Ti Halfblood (malison) communities.  

Bite Attack

Melee Bite Attack: to hit = Dex + Proficiency bonus, reach 5 ft., one creature. Hit: (1d4) piercing damage plus (1d6) poison damage (no save).

When using multi-attack, a Yuan-Ti can use its bite only once  per turn, but can use its other attack(s) in some other way.

Sneak attack damage, that would normally be able to be added, can be added on top of the Bite Attack. 

An off hand melee attack may be made in addition if no bonus action has been taken. 

Shapechanger

The yuan-ti can use its action to wildshape into a Medium giant poisonous snake, or into a Large constrictor snake, or back into its true form. Maximum duration in hours equals 1/4 of total character level (minimum of 1 hour).