Void whale
MASSIVE FUCKING WHALES IN SPESSS!
Seriously though, these fuckers are on average, twice the size of Earth TERRA.
It has been commonly accepted that an entire tyranid hivefleet could NOT eat one void whale.
Once, a large band of space wolves found a hideously malformed void whale drifting through the galaxy. They killed it, and that created a starbirth nebula.
So...
Kill void whale : receive nebula.