Ma'at is order and harmony as well as the creative power of the universe. Magicians often tattoo a hieroglyph of Ma'at on their tongues to help them speak spells clearly; Ma'at is represented by numerous things, including sphinxes. The opposite of Ma'at is portrayed by Isfet, Chaos.
History
Magicians of the House of Life seek to uphold Ma'at. In ancient times, this was done by serving the pharaoh, but with the fall of Egypt, they have now turned to banishing the gods to the Duat in an attempt to keep balance. Ma'at hinges upon the order of creation, including one's loyalty to the rightful king.
The Kane Chronicles
The Red Pyramid
In the First Nome, Zia puts a tattoo of Ma'at on Carter and Sadie's tongues. Sadie remarks that it tastes terrible.
The Serpent's Shadow
Sadie Kane is able to summon the power of Ma'at in The Serpent's Shadow, which Isis, in response, deems amazing, saying that the last time someone managed to summon the power, the person was wearing a fake beard, as it was a female pretending to be a male (Hatshepsut). Through this, Sadie manages to view everyone in their Ma'at forms, in particular, Walt, who was wrapped up in linen, and making her realize he was already marked for death, scaring her greatly. Though weakened through the summoning, they manage to defeat Apophis' minions in the museum, and escape. Later, during the battle in the Hall of Ages, Sadie summons Ma'at to stop the Hall collapsing, succeeding and even repairing all the damage to it.