Kill Morta'gya the Keeper and obtain the Tablet of Shadra.
Shadra the spider loa... the venom queen, a spirit of darkness and death! If there's been a sacrifice, they're trying to bring her forth as a last-ditch effort.
If the Vilebranch are performing rituals, I'm not going to let them finish.
Check the cave behind the altar at the top of Jintha'Alor. Deep within it, one of Shadra's keepers might still be lurking. Kill her and take whatever she carries back here so we can know what's going on.