To get this trophy, you must get all the lightstones and all the spiritual gifts AND THEN fight the Godslayer once again. Only then will you get the extended ending.
To get all the spiritual gifts (all of which have their own trophy in the list), you must get all the lightstones (there are 9 per region) and return to the place where the region's boss battle was. Place the lightstones in the gold archway there and the light creature will return, follow it (this part is easy) and it will lead you to a hidden skeleton. Once you've arrived, walk up to the creature and you can collect the gift.