In the cutscene where The Witness yells at Calus, a large shadow goes over him and he looks upwards in terror as it says "Secure the veil. We will create the link."
Additionally, The Witness' voice sounds a little different.
What are your thoughts on this?
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The Witness was definitely angry enough to show its true form to Calus if thats what it was. Also, during that cutscene, Calus starts bleeding from the head after the Witness yells at him, maybe he uses that avatar so it doesn't kill his disciples? Just a thought