Not sure quite yet. Light fall is supposed to be the end of the light/dark saga I think. Xivu is the last of the 3 siblings we have yet to kill. Currently reading book of sorrows again. She might be tied into it but like the Witness is supposed to play part in the next dlc. The worms that are in pact with the siblings are agents of the darkness. I feel like if the next dlc doesn't have something to do with Xivu it'll be disappointing.
English
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The final shape is the end of the light/dark saga.