Fortress of the Yuan-ti was published by Wizards of the Coast for Dungeons & Dragons 3/3.5 Ed.
Credits: Sole author, except for the action point system, which was modified from my original point system for defeating the Forgotten King.
Special: In this adventure, the PCs have multiple opportunities, through various victories and objectives, to interfere with the Sacrament of the Risen Abyss, a potent rite conducted by the Vanguard of Sertrous. Depending on how well the PCs do, they gain advantages against the Risen King, an undead monstrosity against whom they would normally stand no chance.
As published, Fortress of the Yuan-Ti represents this weakening through a system of action points. The PCs gain action points for their actions, which they can then use in the final battle against the Forgotten King. Presented here is an alternate system, perhaps slightly more complex, but which some DMs and players may find more flavorful and appropriate for the scenario as written.