I've reached the temple where you fight many shadows... I've already found two pieces from the golden disc (one after the statue's test and second from a chest) but i could not find the third part. Also i've tried to open a door near the sanctuary (where you have to walk down the letters to form a word) but i've failed. I have a key or something (shadow prison) but is no use.

If memory serves, there are two doors behind the letters thing. I went through the left one first, did whatever had to be done (just can´t remember) and then had no trouble opening the right door. The missing piece is in there.

Survivor

I just did that yesterday...
You spell out that prophet child's name on the blocks, then go thru the door on the right, and there is a shade there who asks for help...In that same room, look in the top left corner of the room. That is the other piece.