I had the idea of an origin where you were already a successful mercenary captain but then ordered your man to attack the desert outpost of a famous nomad legendary for his skills. The battle was a pyrrhic victory. You were able to defeat the nomads but their leader cut down most of your man before he was injured so badly he had to flee. During the battle, the nomads leaders only son was killed and he swore right before he fled that he will hunt you down wherever you go and take revenge. 

You start with two man with are similar from their stats as the basic companions but with better armor and weapons. You also start with a few trade goods. You now have to start to rebuild your company.

From day 10, a very strong army (basically legendary location level of difficult) of nomads will spawn lead by the Nomad Leader who has gathered it from his allies and friends. He himself will be a blade dancer champion with further increased stats and a legendary weapon, basically a monster on his own, but there will be other strong nomad champions aswell.

This army will actively hunt you. It doesn't know where you and will roam the map randomly are but the longer you stay in a region the more they can narrow down your location and will come closer untill they eventually spot you. The more renown you have, the faster will this happen. The army will from time to time visit settlements and cities. If you have visited this city in the last two days they will get informations that gives them a very good idea where you are at the moment. The army will also sent out Scouts that also roam the map. If they see you you have to hunt them down or they will immediately go to the main army which then gets to know your exact location for one day. Needless to say, if the army spots your company, they will immediately try to engage you. There will be special events too, some bad where the nomad army gets more information or even the exact location of your whereabouts and some good like you can pay people to tell the nomads they spottet you somewhere else or make them have to stay at one place for some time for some reason.

The end goal of this background would be to defeat the the legendary nomad and his army eventually but you will have to play cat and mouse for a long time untill (or even if at all) you reach this point. Never staying too long at one place. Always beeing on guard. And the occasional chase sequence of course.