Parry: can add +2 to an attack that would hit does this 3 times a day

Ability Variants can be chosen for each creature but a creature can only have one of the following

Chameleon: Gains the ability to change its bodies and armors color to it’s environment giving it adv. On stealth checks

Blinding spray: Gains ability to use the poison spray to blind a target for one turn on a dexterity save of 16.

Damage shed: Once per long rest the yuan-ti can shed its skin healing up to 3d10 dmg 

Pit sense: gains the ability to see emitted heat from creatures and gain adv. on perception checks and the darkness spell doesn't obscure its sight.

Multi attack: can make 3 attacks with its two handed Scimitar with a reach of 5 feet, a  +6 to hit and doing (2d6 + 6) Slashing Damage.

Poison spray: as an action can spray poison from their mouths in a 10ft line any creature within it must make a con save of 17 or be poisoned taking 2d8 poison dmg

Sseth's Influence: On its turn as a bonus action can target 4 creatures that look in its eyes on a wisdom save 18 on a failed save the targets are put under a trance that the Yuan-ti can manipulate their vision to cause warping and hallucinations. The creatures when hit with an attack are considered surprised and grants advantage. Lasts for 1 minutes Can be used once per short rest.  Creatures must make wisdom saves on each of their turns after, they can only stop this effect on a successful save.