According to myth, who defeated Apep the serpent every night in the underworld?

[Apep/Apopsis] was the ancient Egyptian deity who embodied darkness and disorder, and was thus the opponent of light and Ma'at (order/truth). He appears in art as a giant serpent. Ra was the solar deity, bringer of light, and thus the upholder of Ma'at. Apep was viewed as the greatest enemy of Ra, and thus was given the title Enemy of Ra, and also "the Lord of Chaos".

Apep was seen as a giant snake or serpent leading to such titles as Serpent from the Nile and Evil Dragon.

While in most texts Apep is described as a giant snake, he is sometimes depicted as a crocodile.

Tales of Apep's battles against Ra were elaborated during the New Kingdom. Storytellers said that every day Apep must lie below the horizon and not persist in the mortal kingdom. This appropriately made him a part of the underworld.