The Fist of Horus is a combat magic spell that summons a huge energy hand.
This spell can be summoned and formed in different ways. One form is summoned by the use of the divine word, khefa, and creates a blue fist that slams in to the target.
Another is to raise one's hand (without needing to utter a divine word) and an energy fist slams in to what the castor extends his/her energy, too.
The last one is summoned the same way as the second, but instead of summoning a fist, the Eye of Horus burns against the target, which acts like it has been punched by a giant fist.
During the attempt to get inside Set's pyramid Carter Kane uses the spell to slam a door shut, before sealing it. Later, he uses it against Set himself, in a moment of union with Horus, through pure rage.
Carter had not been able to use this spell ever since he had separated from Horus, who had been using Carter as his host. But during an escape, using a similarity with the war god, Carter is able to summon it to blast a door open. He later realizes that tapping into a god's emotions is the key to divine magic.
While trapped inside the petsuchos, Carter prepares to use the Fist of Horus to free himself but is inadvertently rescued by Percy Jackson. During the argument that follows, Carter accidentally casts the Fist of Horus and flings Percy into a nearby swamp. Carter initially thinks he killed Percy with the spell, but Percy survives as he's a demigod and the accident leads to a fight between the two that ends in a draw. After calming down, Carter apologizes for using the Fist of Horus on Percy, telling him it was really an accident.
After Setne taunts him, Carter summons the Fist of Horus above him and smashes Setne into the ground so hard with it he's completely buried.