How to unlock hollow knight/pure vessel in hall of gods

[Just_Void]

I assume you have to beat pure vessel in the pantheons to unlock him but I am not sure. Is this correct? Or do I have to beat the normal radiance first?

[Nurtemms]

You need to see him in the Fourth pantheon at least once (you do not have to beat him, just get to him) or after you complete the three pantheons you unlock godseeker mode. In that mode he'll be available in the hall of gods regardless of whether or not you've seen him before
You do NOT have to beat the radience first