Fading is the only way that gods or Titans can die. It is caused by diminishing of their realm or people not believing in their principles. Gods that are not well remembered or were never worshipped well can also fade. Gods are able to resist fading to a certain extent, but it eventually overcomes them. It appears that belief in oneself can have psychological effects on fading. Demigod children of Hades can detect the fading of gods, or perhaps just when a dead or semi-dead one is near.
Fading gods can give a portion or all of their powers to a mortal just before they fade.