This lore was found among the conjurer's tools fat the site of Magusar's summoning in the cave beneath Dooptopia
A Gods power to influence the world, and to sustain their continued existence. is tied to the number and devotion of their worshipers.
The effect of the followers level of fervor is extremely significant. As such, a god with a small following of fanatics can overpower gods with much larger followings.
Gods who fade from mortal awareness entirely will lose all their powers and eventually die. When this happens, they typically are reborn in the Abyss as tan'ari (demons).
In the event of a gods death, their portfolio of influence is passed on. Typically this portfolio is inherited by the deity of a related domain, but interlopers and even non-deities are capable of seizing a slain gods portfolio; thus becoming gods themselves.
A handful of mortals have managed to ascend to divinity in this way, but most died or went mad, sometimes within minutes of ascending. Mortals that were appointed by other gods fared a bit better.