I try to learn by trial and error instead of doing a lot of reading as I enjoy the process of discovery but sometimes I make exceptions for the sake of expediency.
After dragging suspicious marble around for days I reconstructed the three set pieces. Waited for a full moon to see what would happen and they came to life and dropped sketches after they were dispatched. I rebuilt them at my base and one night I heard a loud thunk sound so I went to investigate, all three pieces were shaking (happened to notice it was a new moon, aha). The battle was way more than I bargained for but I survived and a shadow atrium was dropped. I also have the key.
So what now? I know something of reconstructing a skeleton (how and with what?), a gate somewhere and an atrium (how do you get there?). This is my last boss to do solo and after reading a little just getting there seems difficult, upside down tentacles, permanent insanity, and hostile creatures everywhere.