After you enter Tartarus for the first time (when you overhear Xenia and Niobe arguing), if you turn around to leave by the same red door you came in by, you'll find it's locked. With a key? No! With a puzzle! Like all normal doors in the real world!!
Sorry. Anyway, you need to solve this puzzle full of hands to open this door and go back to the ferryman. Read to the end of this hint for a step by step.
If you mess about, you'll discover that when you click on a piece with no hands on it, any hand pieces that touch your piece will be gathered into that piece.
The goal here is to gather all the hands together so that eventually, you can click on the piece with three hands on it and gather all the others in, leaving the board empty of hands. To do that, you need to make them cluster around that three-hand piece.
This is a little like herding animals. You don't want to click on the pieces themselves; rather, you want to click where you want the pieces to go, and they'll scurry over by themselves.
Plan ahead, or you'll end up with one piece sitting out by its lonesome with no way for it to be gathered in. You might want to click first on one side, then on the other, to gather all your little chicks together.
You can only click on the three-hand piece once, so don't touch it till all the other pieces are clustered around it.
Be prepared for lots of trial and error until you find the right way to do this, especially in the master game --
-- or you could just look at the pictures here and whiz through it: