The Kane Chronicles/WMG

The Faust family is descended from the Faust.

The universe of this series fits All Myths Are True and at least some elements of the Abrahamic religions are shown to be true (Moses is said to be a powerful magician). So it's possible that Mephistopheles exists in the story's universe as well.

In-universe Lauren Faust is a wizard.

She is a wizard, but never used any ancient knowledge on her works beyold the point of reference, all her doings such as the creation of My Little Pony Friendship Is Magic are pure skill.

Related to above, in-universe brony comunity is partially aware of Lauren Faust being a wizard.

This is treated mostly as a meme by the comunity and no one actually takes it serious, however, the few bronies who too are wizards have a deep respect for her.

The books' titles relate to their endings.

• The Red Pyramid has Set on the loose with Apophis on the rise, while The Throne of Fire ends with a weak Ra and a preparing-to-escape Apophis, and soon there'll be The Serpent's Shadow.
• Now, in Egypt red is a color of chaos while black is a color of order. So the first two books with their cliffhanger endings have titles that evoke that color (Red is red, and fire is usually depicted as red) while the third one evokes the color black (shadow- black). This hints at an orderly, happy ending to the trilogy.