Well, instead of fighting the Yaun-ti, the party negotiated with them. Ussal, Yassah, and two broodguards were sent with them to look after the Yuan-ti's interests. (Mostly find and destroy the soul monger to make way for Dendar the Night Serpent). You were probably picked by Yassah in a "you two come with me" kind of way.How did the yuan ti join the party? And did the broodguards come voluntarily, or were they ordered by the malison?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.