So this season we are collecting artifacts right? and many describe that inside the relics are body parts. The Witness is making Eramis collect them, but why would The Witness want body parts? Well, I think I know whose body parts they are and now I think I understand why The Witness wants them. We got a glaive last season called Nezarec's whisper. We also recently learned how Nezarec was a disciple. The Drifter has apparently visited the fourth tomb of Nezarec, and the Drifter also said he has other relics like the ones we are collecting, I believe he got them from there. I believe The Witness is trying to recollect the pieces to rebuild one of their disciples. I believe The Witness is trying to ressurect Nezarec. This next part is a bit of a stretch, but... Nezarec is described as the final god of pain by some and based off that description I believe The Witness will use Nezarec to achieve The Final Shape.