The Yuan-Ti have decided to conquer earth and preserve its ecological diversity before mankind once again rises to an advanced technological civilisation and destroys it. This will preserve with the genetic material they need to artificially supplement their small genetic pool to prevent cloned Yuan-Ti from degenerating into mindless snake men through mutation. They will cull the population of humans, elves, dwarves and goblins (which they see as one species) to keep it manageable. They need some human figureheads to make this process more easily palatable, and because the Yuan-Ti are a non-hierarchical, non-territorial society that does not operate on an exchange economy, while the most widespread societies on earth are. Also, the Yuan-Ti are not inherently violent (they are predatory but not territorial) and wish to harness the sentient races who are to do the dirty work.
This session was a lot of roleplaying. There was a brief period where the PCs were taken to the Yuan-Ti outpost, and then they met the 2000 year old Yuan-Ti Ambassador, who had silver translator wires that let him speak Emern and Tash (and all other human tongues), except for words that didn't exist in each language but did in Yuan-Ti. The voice his translation wires projected was metallic, like Stephen Hawking with emotion, until they had trained him to speak the languages. When it couldn't find the right word, it spat out "UNTRANSLATABLE" like Stephen Hawking on PCP. He looked mostly human except for yellow snake eyes like a python. They ate a bowl of live baby mice and drank a diluted narcotic venom in cut crystal goblets in an art deco lounge. One of the PCs had to pee, and managed to get through using a modern toilet without humiliating himself (the other side of the room had a trench for the more snakey Yuan-Ti to slide into for snake craps). The Yuan-Ti let them stay in the guest suite of their cave-fortress's tower, which had a computer, a printer, a Brita pitcher and a handheld neural-disintegrator along with several comfortable beds.
Most of the session was talking to the Ambassador and trying to figure out his agenda or see what they could get from him. He gave one PC an injection that stopped the ageing process and cured a minor genetic flaw that would have led to pancreatic cancer in forty years. Another PC found the lost city of Zancalla on the satellite feed, but the Ambassador warned him away from it, saying that it was the laboratory where the underpeople (including the Yuan-Ti and minotaurs) were created, and that it had defenses that were still intact and would kill him. The PCs eventually decided that they would be in deep shit if they accepted, and they had Sapporo Saki, whose player just bowed out, draw out the Resolver of Conundrums, a small white metallo-ceramic disk with a peace symbol on one side and a line on the other which they found in a altar last session that was being protected by a giant undead sloth with laser eyes created during the underpeople wars (or so the Yuan-Ti claim). Nobody is totally sure what it does. Saki concentrated on the disk, clapped both hands around it, there was a fade to white, and the session ended.