Draconic pantheon

Draconic Pantheon is one of the oldest, if not THE oldest.
Babylonian Pantheon is one of the oldest human pantheons, and if you say they are just the continuation of Sumerians, they can be the oldest human panthon.
It is logical that these two pantheons have relations that predate other pantheons. Babylonian and Sumerian pantheons are usually early civilization pantheons. When their believers start establishing cities, rest of the world is pretty much wilderness. Elves and dwarves are on their own lands and lands in between are occupied by savages, monsters and powerful dragon tyrants. Protecting their cities from these evil dragons, and co-existing peacefully with good dragons, is probably a major issue. However, babylonian pantheon may prefer to take the issue to a draconic deity like Bahamut, (because trying to deal with all evil dragons by themselves can cause a face off with the entire draconic pantheon), and ask him to help out in keeping Tiamat at bay. Bahamut's code of honor demands that he protect the weak from Tiamat's minions, and he helps out the Babylonians. In time, Babylonian believers do not forget this service and start revering him in one of his aspects, and call him Ap-Su. So, in a long while (like a few millenia), both Tiamat and Bahamut becomes honorary members of the Babylonian Pantheon.
Sumerians have Marduk, who is also another anti-Takhisis power. I do not know of any further resemblances with Bahamut.

Killing a deity may just mean banishing his belief and his church from the crystal spheres where the Babylonians are worshipped. In ages past, Marduk and his followers led a campaign against Tiamat. Other Draconic deities, such as Bahamut, interfered or were wiped out along with Tiamat as a result of the conjuncture. For the babylonians on that crystal sphere, Bahamut & Tiamat are effectively dead. However, since they are worhipped multiversally by dragons, just the babylonians aren't enough to send them floating to Astral.

I kind of like to imagine that every respectable pantheon has total control over at least a few crystal spheres, making these their primary source of power. But ofcourse such spheres are the minority, majority being "crystal spheres of conflict", a place where most major pantheons have influence, along with a number of "upstarts" and some representatives of other respectable pantheons. In the former kind of speheres, where several other pantheons were wiped out, mythologies are much more radical, to the point of schovenism. In the latter kind, these pantheons still have to co-exist so they tone down the extreme parts of their myths (such as Marduk not killing a dragon deity, but a protecting its subjects from dragons).

Similarities between Nordic and Celtic and Orcish, elvish, dwarven pantheons are also easy to find. I think the reasons are te same.