Just some notes to myself this time:
1- Make sure to use the halfblood archer from the Slayer's Guide as an encounter since you have a decent picture.
2- You can use the maps from the Fortress of the Yuan-Ti whole cloth or as guides for the large temple.
3- Klang from TaleSpin (For Whom the Bell Klangs) could be a human-headed abomination in charge of a group of purebloods who are seeking the Valley of the Sun (if you want to go with the option that the yuan-ti are looking for it as an offering to their god). As per the episode, he disguises his snake-body with robes and a cloak, allowing only his face and hands to show. However, his arms are fake; they are either grafts from another creature (demonic grafts from the Fiendish Codex sound good) or chaositech (if you want to bring a little Ptolus into the game). His movements are not natural for a walking creature so some hints may be dropped about his heritage that way (although with demon grafts the players might think something along those lines, which is good).