I can't for the life of me find the final Codex in Mysteries of the Frost Back Basin-
I still need " The True Purpose of the Crater:"
Anyone have any idea where I can find that?
"On a terrace surrounding a Tevinter statue, west of the Frozen Gate"
The "statue" is where used to be (if closed) a fade rift. There is an annex terrace to the squared terrace surrounding the "statue". The codex is on a table on this annex (you have to jump a short wall to access the annexed terrace).
[Edit]
Some screenshots to help (this one was the trickiest one for me too):