I'm going to draw a parallel between NnT and Bastard!! since they appear to have some elements in common.
To me the Goddess clan represent the angels, while the Demons represent well the demons and Satan is the demon king.
The Goddess clan have even an angelic appearance so the comparison stands.
In Bastard!! the angels initially appear to punish humans, the main character dies and is transported to hell, as it turns out he's the antichrist and the son of Satan. Where I'm going with this you might ask. Well the demons (in Bastard!!) were also sealed. Sounds familiar?
Anyway the way the angels and demons are portrayed in Bastard!! is that the angels have a very extreme and warped sense of justice, which involved them killing countless human lives including those of the "friends" of the MC. That is why the MC hates them actually. The demons are no better, they are particularily cruel and want to usher a "negagenesis".
So while by all means the demons in NnT are bad, they want to exterminate humans, but I think the Goddess clan fall under the extreme and warped sense of justice type of character. So they too could have blood on their hands.
As to why the Goddess clan and the demons have been having this ancestral feud, once again I cannot help but go back to Bastard!! and look at the explanation there. Demons are fallen angels in Bastard, and at their head is Satan. So in NnT too I can see the demons being fallen Goddesses and having been tempted by the equivalent of Satan, aka the demon king. Might even be he's called that, or Lucifer.
In terms of power, both Satan and the demon king could have unparalled power, this is reinforced by the fact that the DK made the decrees.
So to the Goddess clan the demons could be seen as monstruosities and unholy, and more than that their feud has to do with who will claim the world for itself. And just like in the past the humans will be caught in the middle, and some naturally siding with the Goddess, while others with the demons. Heck the demons even have someone who can typically force those who turn his back to him into submission and a forced state of adoration.
We could also explore the Christian themes in the manga, to further establish this parallel. For example the demons have their commandments and while this is something that would make them closer to Christianity, when they're well demons, the Goddess clan would represent Pagans, which is further reinforced by the fact that they use the Horn of Cernunnos, who is by all means a Pagan god.
So there's clearly a Christian theme going on, and this feud could represent the one that the Church had with Pagans in the past, in the era the Arthurian legends take place in. Also it might explain why the demon king possibly broke from the Goddess clan if we take the fallen Goddess route. Having something like the 10 commandments would make him closer to God, than actually Satan or the Devil. So basically the demon king had a God complex (just like Satan in Bastard!!) and he broke from the Goddess clan and established the demon one. Assuming there was only one demon king, and not more.
So in conclusion the hatred the Goddess and demon clan have for each other runs very deep if it turns out the DK was a former Goddess. And that they saw him as an heretic.
I do not think the Goddess clan were sealed in objects,the Horn is by all means a way to communicate with them, so it's not the same as Denzel's swords which btw are druid swords. Basically the Goddess clan lost their physical bodies, because they exhausted their power at the end of the war. That doesn't mean death to them, as they prolly always needed the AoTGs to manifest on earth. Once again in Bastard!! angels are spiritual by nature, while the demons have physical bodies. So the angels/Goddess clan can still exist without a physical body.
When the Goddesses aren't possessing someone, they can decide to reside in an object.It's basically the same as a genie, but it's because they have lost their physical bodies.It's like an intermediate state, I doubt they're really sealed in objects scattered throughout Britannia. I see it as Denzel summoning the Goddess in his sword first, as it wasn't meant for him to use the power now. So when he's not using the Goddess power, she resides in the sword. If after this the Goddess disappears completely,and doesn't return to the sword, then that means she's not sealed inside the sword. And who could have the time and power to seal each and every Goddess inside an object? Sorry it makes little to no sense.
Ban's comment was just about her presence and how he recognized it, which just proves that it was indeed a Goddess speaking to Ban through the horn. It doesn't actually mean that the Horn itself houses a Goddess.
Anyway those are my thoughts atm.
