If a god is severely weakened, he loses control over a part of his portfolio. If he dies, the whole portfolio is free to take.
It will be taken by someone, sooner or later (and rather sooner). But, in many cases, not be the god's enemies. If a deity is to take a part of someone else's portfolio, it has to be, conceptually, "near" his own. A god or goddess of predators and hunters may easily take over the rule over the wilderness; a god of clockworks may not. After destroying an enemy with drastically opposing domain all one can do is to guard the remains so for some time nobody may take the portfolio. That it why, as I imagine it, it is gods of similar domains who war most violently - they have most to win.
Sooner or later the portfolio will be taken by somebody. It does not have to be a god. If, in the mess that results from deity's demise, a mortal will take some control over what happens (like, in your case, by uniting the warring cities under his rule, or by finding a way to prevent the plagues caused by nature's instability), it is very probable that this mortal will ascend to godhood and get an appropriate portfolio.
Last thing to remember is that a deity shapes what it rules over in its likeness and is also shaped by the portfolio it acquires. If it gains society as a part of its portfolio, it will focus on maintaining and growing organized human groups - but whether they will be strong, just monarchies, republics focused on personal freedoms, slaver oligarchies or warlord-ruled tribes depends on the deity's profile.
It will be taken by someone, sooner or later (and rather sooner). But, in many cases, not be the god's enemies. If a deity is to take a part of someone else's portfolio, it has to be, conceptually, "near" his own. A god or goddess of predators and hunters may easily take over the rule over the wilderness; a god of clockworks may not. After destroying an enemy with drastically opposing domain all one can do is to guard the remains so for some time nobody may take the portfolio. That it why, as I imagine it, it is gods of similar domains who war most violently - they have most to win.
Sooner or later the portfolio will be taken by somebody. It does not have to be a god. If, in the mess that results from deity's demise, a mortal will take some control over what happens (like, in your case, by uniting the warring cities under his rule, or by finding a way to prevent the plagues caused by nature's instability), it is very probable that this mortal will ascend to godhood and get an appropriate portfolio.
Last thing to remember is that a deity shapes what it rules over in its likeness and is also shaped by the portfolio it acquires. If it gains society as a part of its portfolio, it will focus on maintaining and growing organized human groups - but whether they will be strong, just monarchies, republics focused on personal freedoms, slaver oligarchies or warlord-ruled tribes depends on the deity's profile.