There is balance between the two.
Yin and Yang is resource in the world of Battle Realms, alongside Rice, Water and Horses. Yin (Black) and Yang (White) is a symbol of balance to the clans in Battle Realms. Yang affilates with Dragon and Wolf comprising as the heroes. While the Yin that affilates both Serpent and Lotus are the villains.
Overview
The four clans of Battle Realms use the power of Yin or Yang to upgrade their units, strengthen their warriors, summon Zen Masters or to unleash devastating magical powers.
However, Yin and Yang can only be acquired in-game through battle; there is no other way to gain the precarious power of Yin or Yang aside from killing one's enemies. In addition, the acquisition of Yin and Yang happens much quicker should one fight enemies further away from home... in short, the further away from your base you fight your enemies, the quicker you'll acquire Yin/Yang points. Aggressive behaviour is thus rewarded in the game of Battle Realms.
All clans in Battle Realms follow a particular side of the Yin and Yang symbol; those following the way of Yin (Dark) tend to be self-serving, destructive and prone to wicked deeds, while those following the way of Yang (Light) tend towards deeds of harmony, selflessness and honourable behaviour.
The Followers of Yang (Light)
The Dragon Clan: The Path of Righteousness
The Dragon Clan prizes honor above everything else. They are Yang followers with honor as their symbol. Their warriors seek to become one with their weapons, and train endlessly for that purpose. Due to their devotion to honour, the Dragon deity assists them during times of great peril.
The Dragon Clan is an ancient and powerful clan; before the Horde arrived the Dragon Clan boasted of having the most powerful (and numerous) armies, alongside the wisest and most learned of all scholars in the land. Indeed, if the Horde had not arrived on their shores, the Dragon Clan would still be the mightiest in the land.
After the arrival of the Horde, however, the Dragon Clan was shattered; honour had not stopped the monsters, and even the bravest and wisest of soldiers and scholars alike could not halt the advance of the Horde. These days, the ways of the Dragon Clan are lost to the annals of time... but some say that the Dragon Clan might rise again, if only someone were to rejuvenate the old ways...
The Wolf Clan: The Spirit of Might
The Wolf Clan was once a Clan that lived on a far away island in harmony and peace... that is, until an unstoppable and unending storm came to their island and nearly destroyed the island... and the Clan along with it. With their mighty totem, the White Wolf's Skull, the Wolf Clan's Druidess Order managed to bring the Clan to a new land... there, they met the Serpent Clan and the Lotus Clan.
Unbeknownst to the Wolf Clan at the time, the Serpent Clan did not truly trust their savage ways, and in many ways feared them, or at least unnerved... the Lotus' leader, Zymeth, stoked the fears of the Serpent Clan into hatred and paranoia. Shortly afterwards, the two Clans made an agreement that the Lotus Clan would attack and enslave the Wolf Clan for the benefit of both the Serpent and the Lotus as long as the Serpent Clan did not help the Wolf; this led to the near-total enslavement of the Wolf Clan, with nearly all of their clanmembers being forced to mine shale for both the Serpent and the Lotus.
The Wolf Clan leader, Grayback, managed to rouse the Clan into rebellion after decades of imprisonment, and led the Clan to victory against the Lotus warlock and their Serpent lackeys. Now they are free at last, but they have not forgotten the horrors they suffered, nor forgiven the other Clans that distributed it.
The Followers of Yin (Dark)
The Serpent Clan: The Way of the Wicked
This Clan was once the honorable Dragon Clan until they were driven away from a terrible unstoppable Darkness which was defeated by the leader of the Dragon Clan at that time. Tarrant, by using a magical orb, sacrificed himself to stop the endless Horde. Ever since then Tarrant's son led the Dragon Clan and renamed it the Serpent Clan and started doing dishonorable things. Their leader is Kenji (If Kenji took the Serpent Clan Path and not the Dragon Clan Path) and his majordomo and second-in-command as well captain of the Serpent guard Shinja.
The Lotus Clan: The Essence of Darkness
The Lotus Clan was once a Clan following the Yang path but turned to the Yin path. Lotus follow the Forbidden Path which concentrates on Death and Decay. They used dark magics to spite out their wicked ways to become the Tree of Corruption. Ever since the scholars used these scrolls to conjure themselves, they to had a goal to rid of corruption by claiming the Serpent's orb. Their leader was Zymeth and his concubine Yvaine.
The Yin and Yang of Battle Realms.