Here´s what the Grand Finale is all about:

...In Chapter 6 the player has to rally with the lead forces of Avalon, muster every skill and experience that has been acquired in the prior Siege Chapters, and come face-to-face with the most desperate hour of all the western civilizations. The fate of the whole of the continent rests now on this one final conflagration, as the walls of Avalon are finally breached by the invading hordes and all hope boils down to a small band of heroes. Siege of Avalon is a story-rich experience, and this final primary Chapter delivers on the promise developed over the prior episodes...