Aristodēmus in Harpers Dictionary of Classical Antiquities
A descendant of Heracles, son of Aristomachus, brother of Temenus and Cresphontes, and father of Eurysthenes and Procles. He was killed at Naupactus by a flash of lightning, just as he was setting out on the expedition into the Peloponnesus, and his two sons obtained Sparta, which would have fallen to him.
Aristodemus in Wikipedia
In Greek mythology, Aristodemus (Ancient Greek: Ἀριστόδημος) was a son of Aristomachus and brother of Cresphontes and Temenus. He was a great-great-grandson of Heracles and helped lead the fifth and final attack on Mycenae in the Peloponnesus.
Aristodemus and his brothers complained to the Oracle that its instructions had proved fatal to those who had followed them (the Oracle had told Hyllas to attack through the narrow passage when the third fruit was ripe). They received the answer that by the "third fruit" the "third generation" was meant, and that the "narrow passage" was not the isthmus of Corinth, but the straits of Rhium. They accordingly built a fleet at Naupactus, but before they set sail, Aristodemus was struck by lightning (or shot by Apollo) and the fleet destroyed, because one of the Heraclidae had slain an Acarnanian soothsayer.
The Oracle, being again consulted by Temenus, bade him offer an expiatory sacrifice and banish the murderer for ten years, and look out for a man with three eyes to act as guide. On his way back to Naupactus, Temenus fell in with Oxylus, an Aetolian, who had lost one eye, riding on a horse (or mule) (thus making up the three eyes) and immediately pressed him into his service. The Heraclidae repaired their ships, sailed from Naupactus to Antirrhium, and thence to Rhium in Peloponnesus. A decisive battle was fought with Tisamenus, son of Orestes, the chief ruler in the peninsula, who was defeated and slain.
The Heraclidae, who thus became practically masters of Peloponnesus, proceeded to distribute its territory among themselves by lot. Argos fell to Temenus, Lacedaemon to Procles and Eurysthenes, the twin sons of Aristodemus by his wife Argia; and Messene to Cresphontes. The fertile district of Elis had been reserved by agreement for Oxylus. The Heraclidae ruled in Lacedaemon till 221 BC, but disappeared much earlier in the other countries. This conquest of Peloponnesus by the Dorians, commonly called the "Return of the Heraclidae," is represented as the recovery by the descendants of Heracles of the rightful inheritance of their hero ancestor and his sons.
The Dorians followed the custom of other Greek tribes in claiming as ancestor for their ruling families one of the legendary heroes, but the traditions must not on that account be regarded as entirely mythical. They represent a joint invasion of Peloponnesus by Aetolians and Dorians, the latter having been driven southward from their original northern home under pressure from the Thessalians. It is noticeable that there is no mention of these Heraclidae or their invasion in Homer or Hesiod. Herodotus (vi. 52) speaks of poets who had celebrated their deeds, but these were limited to events immediately succeeding the death of Heracles. The story was first amplified by the Greek tragedians, who probably drew their inspiration from local legends, which glorified the services rendered by Athens to the rulers of Peloponnesus.