Surrender

Surrender is the last resort of a faction that seeks to avert its conquest by another faction during war and preserve some of its status.

Codex
"When a faction is weak, its leading house may not be able to hold on to its independence. Rather than being conquered, the house may seek to become a vassal of a more powerful nearby house, but if it is too weak it may not be able to negotiate those terms, leaving surrender as the only option.

When a leading house surrenders its faction, it loses its vassal colonies and status as a leading house, becoming a ruling house within its master's faction. However, it retains control of its home colony, hoping perhaps to return one day to the heights of power.

You will lose the game if your house is no longer a leading house."