You were a noble of Aethel, and too have been gravely insulted or hurt by the usurper king. (Lands stolen, family executed, etc)
You have spent a few years building your forces and subtly sabotaging the kingdom.
When the banished heir starts their invasion, you take the chance and throw in with them
![](https://img.wattpad.com/cover/201963497-288-k687244.jpg)