What is a god? Certainly, they are an embodiment of pure spiritual power, a power that wraps itself in one archetypal nature. They seem to have a sort of symbiotic relationship with humanoids, for the amount of power they can bring seems directly correlated to the amount of "mind" they get from humans. The more attention paid, the more worship given, the more fear, the stronger the god.
Thus, it is strange to consider that a god might die. If they are the embodiment of a concept or a nature, how does that die? The astute reader may already have some theories from the mention of the symbiotic nature above. If the minds of mortals cease believing in a god's nature, could that god die?
Unfortunately, my research has proven it is literally impossible to find a record of any god having been forgotten. The very act of my reading of a god in a history brings it into being! Still, I have read snippets of nameless gods that seem to be of concepts that no longer exist in the minds of men - often gods of specific places that time washed away. As a result, I have concluded that gods do, indeed, die when they are forgotten.
The other way a god can die is much more violent, and this happens extremely rarely - it is absolute spiritual destruction by another god or gods. Mortals do not stop paying mind to this god's nature, but the spiritual mind behind it is gone. When this happens, the nature of the god seems to vanish entirely from the mortal realm. Most recently (200 years ago), Belgot, god of disease, was violently killed by one of the Greater Gods. When this happened, disease and rot disappeared from the world, and have been gone since.
As long as a mortal pays mind to it, however, a dead god never stays dead. Eventually, the focus of humanoids will cause it to reform. There appear to be various ways to accelerate this process - intense worship, certain rituals, and the accursed forbidden practice of Corruption Magic.