There are plenty of canonical material that deals with killing a god. In and of themselves, many gods are originally mortals in one form or another. And that being can usually be killed.
The mantle, or the actual power of the god cannot, however and will usually end up with a new "host", i.e. another being will end up in place of the god that was killed.
Some examples are: Bane, Myrkul and Bhaal. Originally mortal adventurers, they took over the mantle of godhood from Jerkul and divided his portfolios between them. During the Time of Trouble, they were all killed, some by mortals. Eventually, they have returned in some form or other, simply because the mantle of godhood itself cannot be destroyed.
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Well, I suppose Ao, could theoretically choose to destroy the mantle of a god, then effectively kill that god and the power of that god. But mortals will never be able to destroy the mantle of a god.
Edit: Unless you, as the DM, decides otherwise in your game. ;)
Tiamat created the material plane, definetly major God just few worshippers
Raistlan is the GOAT, he is an unstoppable force of nature, he almost ended the world and killed a primordial force of evil, he is multiversal
Canonically, Takhisis is an aspect of Tiamat. Raistlin didn't actually kill her, he temporarily destroyed her connection to the Dragonlance world.
Find your own truth, choose your enemies carefully, and never deal with a dragon.
"Canon" is what's factual to D&D lore. "Cannon" is what you're going to be shot with if you keep getting the word wrong.
There are plenty of canonical material that deals with killing a god. In and of themselves, many gods are originally mortals in one form or another. And that being can usually be killed.
The mantle, or the actual power of the god cannot, however and will usually end up with a new "host", i.e. another being will end up in place of the god that was killed.
Some examples are: Bane, Myrkul and Bhaal. Originally mortal adventurers, they took over the mantle of godhood from Jerkul and divided his portfolios between them. During the Time of Trouble, they were all killed, some by mortals. Eventually, they have returned in some form or other, simply because the mantle of godhood itself cannot be destroyed.
...
Well, I suppose Ao, could theoretically choose to destroy the mantle of a god, then effectively kill that god and the power of that god. But mortals will never be able to destroy the mantle of a god.
Edit: Unless you, as the DM, decides otherwise in your game. ;)