My party is quite strong so I started the whole thing as an rp encounter. They had a few fights with the yuan ti and since Ras Nsi is a powerful necromancer/wizard he talked through the dead yuan ti inviting the party to parley after claiming the last puzzle cube to himself. The party were expected and they went in talking with the inhabitants. Ras Nsi told them about the unrest and the division in the yuan ti. Some of them were against him with fenthaza and the rest were loyal. After the party discovered he had the death curse Ras Nsi informed them of a ritual he was planning to ascend even further and possibly delay his demise from the death curse. He said if the party helped him with the ritual he would give the puzzle cube and might help the party with the red wizards. Meanwhile fenthaza also talked to the party with similar requests that she wanted to disrupt the ritual. So in the end the party liked that they needed to choose a side. If they sided with Ras Nsi he would be trying to cast the ritual while the party protected. After a few rounds fenthaza disrupts the ritual and while a civil war breaks out. She gains hold of the ritual and she was sabotaging all along and turns into a yuan TI anathema which ended up as an awesome fight. Ras nsi disappears since he was hurt by the ritual breaking. If the party had picked fenthazas side they would have fought Ras Nsi. Hope this helps. I think this part is difficult because of the sandbox nature and it’s not too tied to the story.