The Fane of the Night Serpent is a den of vipers. Tomb of Annihilation features this Yuan-ti stronghold. The Tomb of Annihilation PDF details the layout and the inhabitants. DMs use the digital maps to run the stealth or combat encounters. The book also explains the rituals performed there. The PDF helps the DM describe the horror. It also details the prisoner mechanics. The digital guide ensures the faction play with the Yuan-ti is handled correctly. It is a key part of the adventure that leads to the final tomb.