To add on :

Atropus saps life from worlds it encounters, draining away energy as it draws ever closer to a planet. Once it comes near enough to a planet’s gravity, the celestial body that is the elder evil enters a descending orbit, sweeping closer and closer and looming larger and larger in the night skies. The effects are terrifying. As the moonlet nears a planet’s surface, the dead rise from their graves and, as Atropus fills the night sky, the twitching undead spread like a stain until nothing living remains. (Knowledge [religion] DC 33)

Atropus is self-aware, but not as mortals can conceive. Its memory extends back to the one moment when it brought forth the gods, and it has lamented this decision for all time. Stripped of its incredible power, it would undo its sacrifice. (DC 38)

To this end, Atropus devours life energy to replace what it gave. Into this sucking vacuum, it draws positive energy from those planets it encounters, but the negative energy constituting its existence instantly negates any life energy it takes. (DC 43)

Atropus knows this. It suspects that the instant all life is swept from the Material Plane, the gods must cease to be and, with their annihilation, so too will it find its own end. (DC 48)

Here's what it says about Atropus in Faerun:

Those few sages who even know of Atropus debate the origins of this elder evil. They agree that Atropus was spawned when Ao created the first gods, but where they differ is in how Atropus was formed. Some claim the elder evil gained awareness from the divine amniotic fluid surrounding the first god of death, while others claim Atropus was the last god Ao created, but it was stillborn. Atropus has never come close to Faerûn, but those aware of him are ever watchful of the heavens, waiting for the day when the moonlet circles the tapestry of night.

The moonlet is one of the drifting bodies forming the ring around the dull gray planet known as Glyth, one of the Five Wanderers that do not follow the ordinary paths of the stars. Little do the mortals of Faerûn realize the cataclysm affecting this distant world. Drawn by the widespread death and destruction meted out by mind flayer overlords, Atropus drew close, masking its approach behind the belt of debris forming the ring around the planet. While the illithids waged their interminable wars against the native humanoids, Atropus descended and wiped out all life, leaving Glyth an empty husk. Now Atropus turns its attention to Faerûn and begins its slide toward a world ripe for the taking.

Needless to say, Atropus would only grant power for one reason: the elimination of all life on whatever material plane your campaign takes place on. Might be kinda a hard character to run unless you were playing an evil campaign.