The battle is either the freeing of Warunk, the recently freed town (now under a kind of Twelvegods church of Rondra protectorate), where the story takes place (or at least its beginning),
or the huge, enormous Third Demonbattle, which imho should look far, far, far more enomous than this tiny skirmish (compared to the Third Demonbattle).
If you pay close attention, then you see a round thing there, flying through the video, with splinters of it hitting several parts of the map.
This might be a hint towards Borbard's "Demon Crown", which he perhaps was never wearing (the Wiki says he did, I thought otherwise before), which each of its splinters (the Dmoncrown entirely consists of them) representing a close connection and a kind of gift" by nine, I think, archdemons.
This infernal crown was destroyed during the Third Demonbattle, and each one of Borbard's major minions carried away one of these infamous splinters. They grant *huge* powers by the appropriate archdemons !
Now, several of his major minions are dead now (among them the undead dragon Rhazzazor, in short called "Rhazzi" by TDE fans and Xeraan, the money-hungry one), but these splinters cannot be easily destroyed. By oridinary means, they are indestructible. Since then secondary and tertiary minions took them.
So, they still are there, and are corrupting the lnd.
If you can get a google translation, I can recommend these articles from the main (yet semi-official) Wiki :
Borbrad, he is called an "Alveranier", which means he is a god or a half-god, of the banned knowledge :
http://www.wiki-aventurica.de/wiki/Borbarad
The Third Demonvbattle :
http://www.wiki-aventurica.de/wiki/Dritte_Dämonenschlacht
The Demon Crown :
http://www.wiki-aventurica.de/wiki/Dämonenkrone
The Black Lands :
http://www.wiki-aventurica.de/wiki/Schwarze_Lande
The Heptarchs, Borbarad's major minions, each of them possessing a sinle splinter :
http://www.wiki-aventurica.de/wiki/Heptarch
Helme Haffax is a kind of tragic figure among them : He was once the Mittelrealm's far greatest general & strategist, but gave in into a demon pact because he felt he was not honoured enough by the Middelrealm's rulers, most people believe.