Galactus is a living planet created by the Skrulls, in service to the dark god Mephisto. Using his world, they created Galactus, the Anti-Life Virus. Which travels across the cosmos eating worlds, and by the time Earth is targeted at least a 1000 worlds are known to have been destroyed. The living planetoid had a continent-sized cyclopean eye and several planetary-length tentacles which at the tip were as thick as cities. It had a giant living heart in the core, which was protected by colossal humanoids (aka, the Skrull gods).
The planetoid was defended by a field of asteroids which it used as weapons to destroy a planet's defenses and then bombard the world. It released symbiotic parasites on worlds, which ranged in size from monsters the size of skyscrapers to micoscopic creatures. The colossal tentacles would borrow into the planet while the parasites attacked the surface. In the drained the world of life and ravaged it's natural resources. Galactus' planet-side defenses consisted of its monstrous symbiotic lifeforms and the humanoid-like Lava Men, commanded by Lord Kreega a Skrull.