Continue up the stairs until you reach the very top.
Here, you will have to solve a quite unique puzzle.
The point of the puzzle is to manipulate shadows - position the orb just like in the above screenshot.
Then, jump onto the stone block on the right.
The shadow will correspond to the shape on the wall, and you will unlock the passage ahead.
Run down the corridor and follow up ahead until you reach a large and dark chamber.
Go through the passage and jump down to the very bottom.
Keep going up ahead until you reach the chamber with a huge staircase.
Keep going up - there is quite a long way ahead.
This is the end of the first chapter of the game.
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