Where is the third part from the disc (taken from Temple)

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.