The town of Wonderment was populated largely by African-American former slaves and other Non-Whites (including nearby Native American tribes) - this coupled with the fact the town had many nearby resources inspired the villain known as Tarantula to hire a group of outlaws to clear the town at any cost so he could profit from said resources.
Thus the Nightriders were formed, led by the fallen hero known as Kid Cassidy the group were given orders to force the inhabitants of Wonderment to flee or be killed, due to the rampant racism of the era many of the Nightriders were very clear in their intents to massacre the town and lynch any they came across, making them even more of a threat than their already violent commands dictated.
Alerted to the danger many of the Old West's heroes gathered upon the town to rescue it from certain destruction and eventually the Nightriders were defeated and their plans ruined, with Kid Cassidy being strangled to death in the conflict.
Unfortunately the battle against the Nightriders proved to be one of the most costly fights for the Old West heroes, with both Outlaw Kid and Two-Gun Kid falling in battle.
each member of the Nightriders was an accomplished gunfighter and horse-rider as well as a dangerous criminal, many of them were also murderously unstable and thus completely willing to die in battle.
- while often associated with the KKK the Nightriders predate the real-world Klan, they are based on similar hate groups found in the Old West however, who would likely go on to inspire / found the KKK.