How to use the Eye?
depending on which end you are looking for
If you are looking for the purity ending, you must tell him the names of Lukka (you must do his mission related to Echo and all that), Torgar (you must make him reject the corruption 100% and join your team being uncorrupted) and finally Drenth (the dragon that appears as the guardian of Nyx's seal, you must make a deal with him and FOR NO REASON, break him, corrupt him or kill him).
As a recommendation, once activated the power of the pure eye, corrupt yourself to the point of being unrecognizable and go against Nyx, only then you can defeat him.
sorry, I'd like to help a little more, but that's all I know.