During the "Follow the Wizard" quest, you will enter the source temple, it's also then when this quest activated.
Inside, go all the way forward, until you reach a broken teleportation mirror. Go back a little, and get close to a pile of stones, to the North from the waterfall, blocking a passage. Press the switch on the right side of the rubble and go through the new passageway.
At the end of the corridor, you will meet a door, which will tell you, that you need a number of star stones in order to enter, or that you have enough of them. You need to have all the 14 star stones collected. Their places can be found in the "A Source Hunter's Journey" section of this guide.
After entering the room, a lot of doors should attract your attention. You don't have to do anything with them, fortunately, as solving the riddles connected with them will award you with three books, which tell you how to set the candles on the circle at the end.
Leave the doors and go upstairs. Light all the outside candles, and, as soon as the journal is updated, the one in the middle. Finally, light the middle one, the one in the upper right corner, and the one in the lower left corner (so that the candles are lit diagonally). The door should be now opened.