To eliminate a kingdom, you need to kill every family member in every power holding clan in the kingdom. If you don't know how to open the console, I can also tell you how to enable it in files. Download kingdom destruction mod, very helpful, after the kingdom lose all fiefs, after 14 days they will disband, and join other factions, just follow

Upon informing Istiana and/or Arzagos that you have collected all 3 pieces, you must choose to support an existing imperial fragment (path to save the empire*), join a non-imperial kingdom (path to destroy the empire*), or create a new faction entirely (path to save the kingdom if imperial in culture, path to destroy the empire if non-imperial

Can't defeat the empire. My main quest at the moment has me defeating the empire. At first it was bring the empire kingdoms down to less than 4 towns. I did that, and in fact they have no towns and no castles at all. I have now been hunting down every single last one of the individual lords and ladies and at first I was defeating them in battle

Factions (Bannerlord) B. Beni Zilal. Brotherhood of the Woods. C. Company of the Golden Boar. E. Eleftheroi. Embers of the Flame.

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mount and blade 2 how to destroy a faction