The ritual the OP is referring to is actually part of the Tomb of Annihilation hardcover as well as some of the DDHC guide adventures for Tomb of Annihilation.
If you are running players through these it is useful to have an idea of what such a ritual might involve so that the players can have a decent idea of what their character has to do in order to earn the benefits (and possible costs). The ritual is only available to humans.
Volo's Guide to monsters includes a section on the Yuan-ti that adds some details of what can be involved in these rituals.
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Besides the HC, I think it is also story award from DDHC-TOA-10 Ruins of Hisari. The ritual allows a player character to become a yuan-ti pureblood acquiring some of the racial characteristics while retaining their original stats (and presumably starting feats if they were a variant human to start with since the feat replaces stat boosts).
The ToA primer for DDAL states the following about the ritual and factions:
"A character that is a member of any faction other than the Lords’ Alliance (the only faction that permits yuan-ti) that accepts Fenthaza’s offer of transformation is removed from their faction; they immediately lose all ranks and renown with their previous faction, and must start at the lowest rank of the Lords’ Alliance with zero renown. In addition, they gain the following story award: Enmity of the <Former Faction>. You have betrayed your former faction or the promise of profane power. Any Charisma checks made to influence members of your former faction are made with disadvantage."
This may not be the most recent version .. but the most recent version appears to be the same.
http://media.wizards.com/2017/dnd/downloads/ToAPrimerv1.pdf
The consequences of doing the ritual are up to the DM in a homebrew game.
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Fenthaza encounter ToA HC:
"She also offers to ritually transform human characters into yuan-ti purebloods so they can get closer to Ras Nsi." ToA-HC p.114
"To undergo the change, slaves must bathe with snakes in the blood of sacrificial victims while under the influence of temple drugs. The transformation leaves each slave with 1d6 levels of exhaustion." ToA-HC p.119 (one in six chance of dying).
During the ritual, other yuan-ti are eating the slaves that are not being transformed.
The ritual can only be undone by a Wish spell or divine intervention. The results of the ritual if you survive are the following:
Characters who survive gain a random form of indefinite madness (DMG chapter 8). In addition, "a human that undergoes and survives the ritual retains its traits and gains the following yuan-ti racial traits." p 119
Darkvision 60'
Innate Spellcasting: Unlimited Animal Friendship spell (snakes only), Suggestion (once/LR, gained at level 3, charisma)
Magic Resistance: "You have advantage on saving throws against spells and other magical effects"
Poison Immunity: Immune to poison and the poisoned condition.