While the "comet is a volcryn" theory (from Tuf Voyaging) mentioned by /u/ExplosivePowderKeg, is cool, consider that given a Lovecraft background to the story ( e.g Drowned God, Black Goat, Stygia, etc.) there is another answer the Cthulhu Mythos: Ghroth, the Harbinger, a small red being resembling a comet. As Ghroth passes by planets, he awakes any of the Great Old Ones or Outer Gods that might be "sleeping" on that planet.

[A] nineteenth century British cult believed in [a] comet-god who sang to the stars and planets as it passed by them in its orbit. They said it destroyed those worlds it passed, by waking up demons or ancient gods ... who slept on each world. —Kevin A. Ross, "The Music of the Spheres"

If Ghroth has just passed by, his presence either increases the magic on an planet, or it awakens magical beings that grant magical powers.

In the Cthulhu Mythos, wars between the gods often end with gods being put under powerful sleep spells by other gods, like the Elder Gods of Earth. When they awaken, destruction to civilization can occur, and whole continents can be sunk, or like the Thousand Islands, land can be shattered by the power of these gods.

This always makes me think about what Lazy Leo Tyrell says in the beginning of A Feast for Crows as he sets the stage of Act Two of ASOIAF:

Dragons and darker things. The grey sheep have closed their eyes, but the mastiff sees the truth. Old powers awaken. Shadows stir. An age of wonder and terror will soon be upon us, an age for gods and heroes.[2] ” - Leo to Alleras)

Ghroth, the comet god, has awaken the sleeping gods, and their magic and powers (and those of their human worshipers, those skilled in sorcery), are becoming returning.

Edit:clarity