The Red Dragonborn are notorious for being aggressive, prideful, greedy and vengeful. These are not universal traits by any means, but they are common enough for the large red lizards to garner an infamous reputation as mercenaries and brigands.
It is known that 250 years ago, the Cinnabar clan attended Tiamat's gathering, along with several other Chromatic Dragonborn clans.
The Freeze was not kind to the Red Dragonborn. As the world grew cold and descended into chaos, contact between the Clans were lost.
The remains of the Cinnabar Clan settled in Lasthaven. For a time, they worked to try to find a way to reverse the Freeze. But a growing resentment within the Clan led to a rift, and a schism. Most of the Clan moved south to Lasthaven, to become Mercenaries. The remainder stayed in Lasthaven, to continue their research into the Freeze.
All but two of the Lasthaven Cinnabar travelled North, led by Ioun Priestess Cinnabar Epiphany. She believed that she could find an answer to the Freeze, and reverse it. The expedition was joined by a group of Gnomes, Halflings, and Elves. They never returned.
The Kingston Cinnabar have lived up to the Red Dragonborn reputation, operating as brutal mercenaries to the highest bidder.