Void Gun Blueprint?

[Boon: Gay Bitch]

I've found every blueprint BUT the void gun on Dark Castle. I am aware that I have to go down the hole outside of the gate of the castle which I did but all I get down there is a pitch black hole and I've walked around for ages many different times but nothing has ever popped up.

I have a screenshot of my game. The blue things in the corners and centre are from the Fairy Wand. I used them as a guide of where the boundaries are of the room

https://steamcommunity.com/profiles/76561198173476245/screenshot/2310977007054732932/

Does anyone have any idea what to do? I've tried searching and I keep getting told the same thing but it never pops up? Unless I'm missing something?

Thanks to anyone who helps <3

[JayJ]

there should be a fairly small white altar near the middle-ish of the room