Death
For committing matricide, he was pursued by the Erinyes and driven mad, fleeing first to Arcadia, where his grandfather Oicles ruled, and then to King Phegeus in Psophis, who purified him and gave him his daughter, Arsinoe (according to Apollodorus) or Alphesiboea (according to the geographer Pausanias), in marriage. Alcmaeon gave her the necklace and robe of Harmonia. According to Apollodorus, Alcmaeon's presence caused the land to be infertile, so he went to Delphi for assistance. In Pausanias, it is his own madness which drove him to do so.
From there the two accounts generally agree with each other and with Thucydides. Alcmaeon is instructed by the oracle to find a land which did not exist at the time when he was polluted by killing his mother. Accordingly, he goes to a delta of the Achelous river, which was newly formed. There he marries Callirrhoe, the daughter of the river's god. She had heard of the famous necklace and robe of Harmonia, and asks Alcmaeon to get them for her. He complies, returning to Psophis and telling king Phegeus that he required the necklace and robe in order to be purified. Either Phegeus or his sons (Agenor and Pronous) discovers the truth from a servant, and they ambush and kill Alcmaeon. In Apollodorus, Arsinoe, the daughter of Phegeus, chastises her brothers, who put her into a chest and sell her as a slave to Agapenor. Meanwhile, Callirrhoe prays to Zeus that her sons will grow up instantaneously so that they might take revenge on her husband's murderers. Zeus grants this, and Amphoterus and Acarnan meet the sons of Phegeus at Agapenor's house, when they are on their way to Delphi to dedicate Harmonia's robe and necklace there. After killing them, Amphoterus and Acarnan continue to Psophis and killed king Phegeus and his queen, after which they are forced to flee to Tegea.
