Once you become a warrior and wait a while, a black cat named Nightmare will appear up near the tree at the Dens. Click him and he'll request retiring to the farm, type yes and that's how you get farm life.
You cannot go back to BrambleClan, the only way to go back is through death but you will get softlocked if you go to the border with Ashtail. There isn't much to farm life as of currently, Nightmare seems kinda bugged too. There will probably be more once Moss releases the kittypet update.
