In Greek mythology, Megapenthes (Ancient Greek Μεγαπένθης, "great sorrow") is a king of Argos, son and successor of Proetus. He exchanges kingdoms with his cousin Perseus — taking Tiryns while Perseus takes Argos — after Perseus accidentally kills Acrisius. Distinct from Megapenthes son of Menelaus of Sparta.
Adapted from Wikipedia, "Megapenthes (son of Proetus)" · CC BY-SA 4.0
I.Son of Proetus
Megapenthes inherits the Proetid claim at Argos. He is father of Argeus or of Anaxagoras (in some accounts Anaxagoras is his grandson through Argeus), and of a daughter Iphianeira who marries the seer Melampus.
The name marks the grief that runs through the Abantid house — Acrisius' death, the twin quarrel of Acrisius and Proetus, and the shame that keeps Perseus from simply seizing Argos.
II.Exchange with Perseus
After Perseus kills his grandfather Acrisius with a discus, he feels unfit to rule Argos and approaches Megapenthes. They swap: Perseus takes Tiryns (and later founds Mycenae), Megapenthes keeps or takes Argos. Later tradition has Megapenthes kill Perseus — a rival strand the atlas keeps as disputed.
The atlas id megapenthes-proetid is this Argive king alone. The Spartan bastard megapenthes (son of Menelaus) is a separate node.