quote Cheat - Credit to Just a Shadow

Before being able to fight Chaos you'll have to get 4 fragments:

Clear the following mob hunt marks: Wraith ,White Moose and Orthros, by clearing the last one you'll get your first fragment.

Go back to Garamsythe Waterway there you'll have to activate the following swithches, first turn all of them off, now activate the 11th and 4th , now deactivate the 11th and activate the 3rd, finally deactivate the 4th, go south of the same screen you are and you'll find a shining object, the second fragment

For the last fragment go to Nabreus deadlands, there talk with Ma'kleou, he'll tell you he's waiting for his two disciples, now it's time to find those 2 guys. Go to Rabanastre lowtown, to Dalan's house there talk with the disciple he'll explain you the situation, no go to the vacant house and read the letter (NOTE: You've already done what this letter tells you to do, but you still need to read it). The following step is to talk with Filo in the South Sprawl of Lowtown, after doing so go to the streets of Rabanastre. Talk with a woman that standing watching the fountain, next talk with a merchant in the bazaar you will tell you that he sold the necklace to an Imperial, this imperial is in the magic shop, choose the option 1 option twice. Back to lowtown speak with Filo again, now go and talk with the imperial again, you'll receive the third fragment.
Go to the disciple in Dalan's house and give him the 3 fragments you've got, he'll dissppear.

Teleport to Archades, into the magic shop there talk with the other Ma'kleou's disciple, now go to Old Archades west section and talk with Otto. Return with the medallion to the disciple.

Go to Nabreus deadlands. Talk with Ma'kleou, now go to the screen called the Slumbermead, there go to the north west part of the screen where you'll find a hidden path, follow it. When you reach a shrine a cut-scene will play after that you'll receive all the 3 medallions.