There's a dialogue option with Helsik (Act 3) about the Crown:

Helsik: I am afraid that I cannot take you back to Mephistopheles' vaults. That place is now locked tighter than Lord Mammon's coinpurse.
Helsik: Lord Gortash ("The two of you" if Dark Urge) wanted to steal something from Mephistopheles, so I punched a portal into the Archdevil's dusty vaults.
Helsik: You rascals stole the Crown of Karsus. Pissed off Mephistopheles to no end. (Dark Urge)

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Then again, we see that It and the other Mindflayer on the ship (The one fighting Commander Fiery Greatsword) are seemingly unaffected by the Prism's proximity

Probably because it doesn't work that way, the power of Orpheus needs to be actively siphoned, and the Elder Brain purposely released the control exerted on the Emperor when he obtained the prism.

The Netherbrain: Who do you think told the Chosen about the Astral Prism? Who do you think planted the knowledge of Orpheus' power, and the fear of what it could do? When the Chosen sent my thralls to retrieve the Prism - who do you think let the 'Emperor' slip its leash, knowing it would be the one to bring you to me?

After the Astral fight:

Player: How did Gortash and the other Chosen find out about the Astral Prism?
The Emperor: A very good question. One that I have been unable to answer. That Orpheus lives at all is ruinous to Vlaakith - she has done everything in her power to keep his existence a secret. That Gortash and the Chosen found out about it… this is impossible to explain. But it was important enough to them that Gortash sent me to retrieve it.
The moment I found it, I felt a change. My free will returning. I followed the feeling inside - And found Orpheus. I realised what the Prism was for - containment. While my body was within the Prism's bounds, my mind was free. I could resist the elder brain, the Chosen. Better yet, I could plan to overthrow them. All I needed to do was subdue Orpheus and find allies in the outer world. You.