Alien enemy for After Us, O3

Enemy design for AfterUs

O3 is an alien species which civilization owns 3000 planets in the galaxy. But we don't have too much knowledge about them, as all of their territorial space is tight guarded.
They never accept any kind of contact. Their behavior and motivations are unknown. In some battle with other civilizations they retreated even when they almost won the battle. They attack, occupy or abandon planets for some unknown motivations. Someone says they just got what they want and what the interested then they leave, because the civilization has a higher goal.