Ok when you fight the main dragon boss
you can choose to fight or leave if you leave it
will give you an ending and an achievement.
If you fought him and won and got to the part in the everfall
after colecting the 20 wakestones, you meet the Seneschal
you can choose to fight or leave, leaving gets an ending
and an achievement.
if you fought him and won, kill him, once you become God you
should open your inventory find the GodsBane Blade, press A on it
then press use, once this is done notice it says in the bottom
press X, do this for the main ending give an achievement and
leading to a NG+.
Now this is the ending that took me a while to get, when
the Seneschal is on his knees instead of killing him
use the GodsBane Blade on yourself and this is another ending
with an achievement.