>saw awakening randomly got an update and a new "decisive battle feature"
>if you and the other clan are large enough a retainer will suggest the decisive battle, if you accept you get a 6th month truce period where the two clans build up and prepare
>both clans gets to choose the castle they'll siege as a prelude, if the attacker wins then send reinforcements to the main battle
>the decisive battle is 12v12, if the sieges are won it goes up to 16v16
>win the battle and you instantly absorb the entire clan and all their officers.
I really like this, lets you make your own sekigahara and massively speeds up the late game. you also don't actually have to march anywhere so you even castles on the other side of japan can participate.