We have always known…war.
Based on the line that follows, my guess is that this is a reference to conflicts between folk and elves prior to their joining force to become the Aedyr empire.
Seems like an obvious reference to the Aedyr Empire which formed in 2399 AI, approximately 425 years prior to the events of the first Pillars of Eternity game. So, whatever setting this is, it's not more than 425 years in the past.
Turned heroes into queens and kings
And this seems like an obvious reference to the Aedyran system of government; a joint monarchy consisting of a human king and an elven queen or consort. Interesting that the queen is reference first. Could it be that our story is being told from the elves' perspective?
This is where I start to get stuck. Obviously, the annals of Aedryan history are still largely unknown to us, but the next major conflict that we know about after the formation of the empire is related to adventures in colonialism. This could be a throw-away line, but I doubt they would include throw-away lines in a world premier trailer. Remains to be seen if these referenced-but-undocumented conflicts are important to our story. Or maybe I'm too narrowly focused on Aedryan history to see the context.
Now our oaths are lost - forsaken
And you must face the monsters
…our sins have born
Obvious Woedica all over this.
So, after animancy was discovered in Aedyr, it almost as quickly became outlawed by church of Woedica and the monarchy. My guess is that our narrator is referencing some event where overzealous Aedyrans decided that this rule didn't apply to them and opted to study animancy anyway. Hence the skeletons and flaming arrows, etc etc.
Is an oath worth the weight of a crown?
These rebellious Aedryans broke the oath not to study animancy because they wanted a shot at their own kingdom???
That's all I got folks. Your turn!