Couatl

Publisher's blurb:

In this level 8+ adventure, the party is contacted by a mysterious, radiant being and asked to prevent the fulfillment of a dark prophecy. Can the group thwart the Yuan-Ti's plans to create an avatar to their dark god, Sseth, or will death's shadow cover the land and plunge the world into darkness?

This encounter is fully playable as presented using only the Player's Handbook and Monster Manual, but does include some alternative monsters for DMs who own copies of Volo's Guide to Monsters, and alternate spells for those with access to Xanathar's Guide to Everything. It also includes two new monsters: A Yuan-Ti Mind Whisperer/Abomination called a Chosen of Sseth, and a corrupted Couatl called a Darklight Couatl.