It's in your documents folder under games > elemental > units
i.e.
Documents\My Games\Elemental\Units
You'll see your sovereign's name along with some random numbers. It's an XML file. Hold right shift, then right click on the file and go to Open With > Notepad. If Notepad isn't available go to Open With > Choose Program and use Notepad.
The history is the text between <backstory></backstory>, at the start of the file.
