Worldeater

You are the Worldeater; an enormous mechanical planet that eats worlds, and also harbors a civilization of it's own. The people that live on your surface need land, water, atmosphere, metal, and even biological material as food to continue surviving, but you also need those same resources as well. 

Travel through the galaxy and consume whole planets to gain their materials and elements. Turn those into the resources your inhabitants need. Different types of planets have different types of materials to gather.

The civilization living on you also helps to maintain you. They build cities, take care of your organs, and construct massive surface projects that aid you in becoming a massive super planet. However, as their population grows, they also consume more resources. You'll have to keep moving, and keep consuming.

Use the tractor beam to drag distant planets into your maw, but watch your iron and gas reserves.

Expel your own atmosphere as thrust to accelerate through space and escape unstable stars.

Make sure you're safe before consuming using the tractor beam. Eating a whole planet takes time. Once the tractor has delivered a world to your grinders, the process cannot be stopped.