My headcanon for thousand year door is that pre-thousand year door is that the Shadow queen led a genocide against of the other species with an army purely based of dragons/draconic species, demons, ghosts and possibly the undead. So I can say wars were suspected to be gruesome judging by the skeletons in the thousand year door. As for the queen herself maybe she is the embodiment of evil herself who was ummoned by an evil wizard who got killed or some shit during that time. I would also like to think that all happened like 1000 years before she was she was sealed and I probably I think it was somewhere during medieval times possibly during Dark ages.