You and the other players are lords (Lord is the title regardless of gender) of small semi-feudal kingdoms, that are outside a medium-large empire. The Emperor of said empire informed you that dragons were gathering in the Blachrion Mountains, specifically near the Holden pass. The holden pass is the only way through the mountains to the more wild part of the continent, which is mostly un-populated (you think). You were going up to the nearby caves when you hear a song.

https://youtu.be/AvG2NMbA5z4?t=21 (it starts partway through the song intentionally)
and you decide to join the dragons cause. Now you must convince the dragons of your sincerity, and then prepare your land, for the coming war. 
The empire and where you are is the southwestern part of the continent, and the mountain range seals off everything to the north and east. Beyond that is the dragons territory, not much is known about the land beyond, the official line is that it used to be a thriving kingdom a few thousand years ago before the dragons killed everyone and only a few made it out through the Holden pass. The Empire wants the land beyond the mountains.

Before the empire rose, before civilization took hold of the land, dragons were free and lived without a care.

	But then humans being well, humans, they began enslaving/killing dragons, burning their nests, and eradicating their younglings (breaking eggs/infanticide)

	That was during what the empire calls 'The Expansion Crusade'

	That was 1000 years ago, now the empire is preparing to expand again.
	
	And the dragons, fed up with the humans, are now ready to wage war against them, led by a powerful dragon warlord (think Alduin or Parthurnaax from Skyrim).

	Naturally, the emperor--and the empire as a whole--fabricates lies that the dragons are the aggressors. Acting like they are just mindless beasts who will raid your farm and burn your town if given the chance.

	You, the player characters, are vassal lords of the empire.  Though you have been loyal since the expansion war, what you discover in the Blachorian Mountains, and hear from the dragon warlord himself, causes you to become sympathetic.  Thus, you--and the small states you rule over--defect from the empire and join the dragons.



There is magic, there are also dwarves and beast-kin/demi-humans. Some say that Elves used to live in the forests before the Empire expanded. 
As for the terrain, you have the Empire, a massive country with about 3 million miles of territory.
 it has a roughly circular shape except where it encounters the western and southern coastline. there is several days travel between the edge of your territory and the edge of the empire. it was put there as a buffer zone in case anything came attacking you and the empire. There is a decently large forest between you and the mountains (2 weeks on horseback roughly) Your territory can be mostly wooded, nea a lake, mostly open fields. Its honestly up to you