It might be up to the DM, yes. However in this case...
Start of Darkness spoilers:
SpoilerXykon states that his soul isn't in the phylactery when it is threatened at one point. It is still up in the air whether or not Xykon could make a new one. But it would appear that Xykon wouldn't be acting the way he is if he could make a new one so easily.
Thus when the phylactery goes away, Xykon is just one bad day away from Final Death.