Okay, so I have a few questions because this plot sounds kind of confusing. Who are these Hymn and Winterland lords?
Lords of a now ancient land whose names were lost to time (i haven't figured an actual name for them yet, since there's been no interest in the rp). The lord and lady of hymns are the leaders of the westlands and are basically the least combat oriented group of soldiers you'll ever see. The forces of the hymns are mostly a peace-keeping force made up of honorific knights from their church (meaning they're mostly peasants, a handful of priests and a few actual knights capable of handling themselves), who prefer to settle things in non-violent manners, that is until the lady of hymns declares war and marches into the winterlands.
The lord and lady of the winterlands are rich, strong and live in the harsh environment of the frozen northern lands. Their forces are much more massive than that of the westlands and are significantly better suited for battle.
Where does the backstory start and where do the players begin? I'm assuming we start as the queen of the hymns' army is besieging the castle, but I could be wrong. After that, is the victor actually decided beforehand or does the actions of players actually dictate that (depending on the side we're on)?
backstory will briefly cover the relationship between the two territories and their rulers. then it will touch upon the destruction wrought in the first night of the war and then run up to the beginning of the battle at the gates of the last stronghold of the winterlands where you would start as the roleplayer. the victor will actually be decided based on ic performance. it can be an underdog story about the lady of hymns winning an impossible battle alongside her ragtag army, or it can be an example set by the lord and lady of the winterlands as they put a desperate woman in her place.
Lastly, why would the winner 'scorch the lands' and overthrow the king, and leave their identity a mystery? If the Hymn people win, they would logically colonize the Winterland territory and tell everyone that they won. If the Winterland people win, they would force the Hymn people back to their own territory and reclaim their own, and tell everyone they had won. Hell, even if either side decided to overthrow the king, there's no reason why the victor wouldn't be known, as "the group that overthrew the king" would not be kept a secret in history.
In the case that the hymn forces win, they would raze the remainder of the winterlands so that there'd be no possibility of someone having escaped in hopes of exacting revenge, then they would travel to the king and overthrow him since she'd be so high on the excitement of victory. in the case that the winterlands won, they would raze the westlands so that they could never again stand up to them, seeing as how most of the population was in the army the only people left in their territory would be those filled with hate and willing to seek revenge. then the winterlands lord and lady would move to the king and overthrow him to ensure that he wouldn't seize the chance to snuff out the biggest threat to his kingdom. as far as the victor being lost to history, well... one part of that would be revealed in the rp itself and the other part of that is that several hundred years later the kingdom which was overthrown ended up burned to the ground and a large amount of documented history was lost alongside it. eventually, both lands are repopulated regardless of the outcome.