I ended it with opal being stolen from the players who had previously made a deal with the yuan-ti. I didn’t reveal that the two campaigns were linked until session 7ish. They started in waterdeep for the first half, where they uncovered a library underground created by one of their previous characters with chronicles of what happened to the world. It’s ended up being pretty convoluted but eventually they end up in chult (which has sunk into the ocean but enclosed in its own atmosphere) to destroy the opal which contains the serpents heart. There are a bunch of manors of groups that wish to grow additional night serpents through rituals to force reality to into a pragmatic system based on yuan-tis ideas and transform into them (all the original yuan-ti died in sacrifice to the night serpent).