In Greek mythology, Megapenthes is the illegitimate son of Menelaus of Sparta by a slave woman (Pieris or, in Acusilaus, Tereis). Homer shows Menelaus marrying him to Alector's daughter; Pausanias reports that he and Nicostratus later drove Helen from Sparta while Orestes still wandered, though the legitimate succession at Sparta passed to Orestes. Distinct from Megapenthes son of Proetus of Argos/Tiryns.
Adapted from Wikipedia, "Megapenthes (son of Menelaus)" · CC BY-SA 4.0
I.Bastard of Menelaus
In the Odyssey Menelaus is celebrating the marriage of "stalwart Megapenthes, who was his son well-beloved, born of a slave woman," to the daughter of Spartan Alector (Iphiloche or Echemela). Apollodorus names the mother Pieris the Aetolian, or Tereis after Acusilaus.
Because Megapenthes and his brother Nicostratus were sons of a slave, Pausanias says Orestes — Agamemnon's son — succeeded Menelaus as king of Sparta rather than Menelaus' own bastard line.
II.Helen driven out
Rhodian tradition in Pausanias has Nicostratus and Megapenthes expel Helen while Orestes was still pursued by the Erinyes; Helen fled to Rhodes and Polyxo, who then had her killed. Pausanias also saw the brothers carved riding a single horse on Bathycles' Throne of Apollo at Amyclae.
The atlas id megapenthes is Menelaus' son — not Megapenthes son of Proetus, who exchanges Argos/Tiryns with Perseus' line.