Hello, I am currently writing a campaign from scratch and have gotten most of the lower levels completed but I have reached a stage in the story where the characters have caught wind of Yuan-ti infiltrating a prominent city to bargain for and steal several rare and powerful components and artifacts for an unknown reason.

The components are for a ritual to summon their god, Sseth. In this world, Mersshualk is in a deep slumber, and in his absence, Sseth has taken over and unbeknownst to the Yuan-ti, has kept Mersshualk asleep so he could gain the power to slay him and replace him on the pantheon of gods. The characters are supposed to try to stop this summoning but fail, and fight the god Sseth who is a CR 22 God (rather low CR because he is still a new "god" trying to amass more power). I want them to be able to explore one (or two) of the pyramids (Volo's guide to monsters), explore the town, and other temples, and finally, find the circle of Yuan-ti Preistesses attempting to summon the god while other Yuan-ti sacrifices multiple humanoids, causing maximum suffering before bathing the powerful artifacts in the victim's blood, all the while chanting to Sseth.

I have been trying to come up with fun, interesting, and harrowing encounters before the summoning of the god, but I need help with this aspect of my story. I want some cool traps and tricks in the temple that I haven't quite sussed out yet but any ideas are welcome and thanks for coming out of your way to help with this project.

Have you looked up the Yaun Ti on the forgotten realms wiki? There should be a fair amount about them there if anywhere as the FRs are generally the most fleshed out of the published realms. The Yaun ti in FR date back to before the arrival of the elves and dragons as they were the created servitors of the first of the creator races - the Sarrukh. With about 35,000 years of history they have had their active times and their sleepy times  but there is lots of history available with plenty of space to add your own portions as well.

Thanks for responding. I have read a good portion of the Dnd 3.5 book "Serpent Kingdoms" on yuan-ti and watched the 3 videos about their history posted by MrRhexx (great lore videos btw). While these are all great resources that are filled with everything about Yuan-ti, I have not really encountered any information specifically about what traps or guards are located inside one of their temples or what ambushes/traps would the PCs encounter before even reaching the city itself. I have added two new types of yuan-ti: a CR 5 "ranger yuan-ti" that sneaks up to the characters before firing a barrage of arrows and vanishing back into the dense jungle (they are aptly named Junglestalkers), and a CR 12 Yuan-ti "Fighter" called Blackguards who are the champions of the serpent kingdom when it comes to fighting. I don't know if anyone would have any insight on how I could set up a few Yuan-ti encounters before reaching the city or while the PCs are on/inside the temples or pyramids.

Will the ritual just be put on hold until the party is nearby? If the priestesses are attempting to summon this god, then the whole thing would have to be on some sort of timer. I'm not sure what direction you would want to go for it, but potentially having a countdown introduced in some way (ex. "The evil god will be summoned in 2 day's time!") could help add tension and move the plot forward. 

EDIT: having a timer for a dungeon crawl is also a great way to introduce traps and harrowing encounters, because the party won't be able to spend time resting and recovering as much if they know they only have XX hours until the ritual is complete. 

Quote from VanZoeren >>

Will the ritual just be put on hold until the party is nearby? If the priestesses are attempting to summon this god, then the whole thing would have to be on some sort of timer. I'm not sure what direction you would want to go for it, but potentially having a countdown introduced in some way (ex. "The evil god will be summoned in 2 day's time!") could help add tension and move the plot forward. 

EDIT: having a timer for a dungeon crawl is also a great way to introduce traps and harrowing encounters, because the party won't be able to spend time resting and recovering as much if they know they only have XX hours until the ritual is complete. 

 I would probably have the information about when the ritual will take place known by most any Yuan-ti the PCs capture alive but they will have to pry that information about what is happening, and what time it is going to happen out of them. I will probably have a longer time for when the summoning will occur because the players would be pretty far away because the Yuan-ti city is in the Jungles on a big island off the coast of the major continent where they are currently located (think of Madagascar alongside Africa but a little bigger). I think I will try to incorporate this to drive the plot forward. Thanks for your insight. Does anyone have any homebrew traps that I could add to one of the shrine rooms in the temple of the Yuan-ti and what would trigger the trap?

I would recommend heavily using poison traps with easy trigger mechanisms. Yuan-ti have the poison immunity, so they can literally poison anything and everything and it'll only hurt invaders. Poison gas, poison needles, poison on door handles, on the toilet seat...the sky is the limit.