If they follow into the mountain their Lantern of Revealing will let them see through the magic barrier that surrounds Pahada Sskati.
They will be approached by Azhih the shaman and a warrior called Ikhass. They will say that these strangers must be taken to Yoahshuss, the king's advisor. If they resist they will say that they will send everyone after them and they will never make it off the mountain alive.
The advisor will question them about the defiler Rowon. They want to know anything about him. He will think it safer to kill them than let them tell the world about the city.
The Yuan-ti will not tell them immediately that the defiler stole a powerful artifact - a magic item that warns them of extreme danger - and then disappeared heading north. He has stolen Bracelet of Dire Warning. An artifact belonging to the Yuan-ti.
When they protest their execution - they will be taken before the king Anu. He will decree that they will be accompanied by Yuan-ti warriors and if they kill the defiler they will be set free with their memory wiped.