Iasion

Iasion was originally a mortal-lover of Demeter, but Zeus discovered the pair copulating in a thrice-plowed field during the festivities of the marriage of Kadmos and Harmonia (a foster-daughter of Iasion's mother Elektra). Zeus, offended by the sight of a mortal mating with his sister, struck Iasion dead with a thunderbolt.

The Mysteries probably expanded on this story and represented Iasion as a spring-time consort of Demeter, who returned to the upper world every spring in the company of Demeter's daughter Persephone.

"Demeter, bright goddess, was joined in sweet love with the hero Iasion in a thrice-ploughed fallow in the rich land of Krete, and bare Ploutos, a kindly god who goes everywhere over land and the sea's wide back, and him who finds him and into whose hands he comes he makes rich, bestowing great wealth upon him." -Theogony 969

"Elektra was subject to [Zeus] the dark-clouded Son of Kronos and bare Dardanos [text missing] and Eetion [text missing] who once greatly loved rich-haired Demeter. And cloud-gathering Zeus was wroth and smote him, Eetion, and laid him low with a flaming thunderbolt, because he sought to lay hands upon rich-haired Demeter." -Catalogues of Women Frag 102 (from Oxyrhynchus Papyri 1359 fr. 2)

"Kalypso shuddered, and her words came forth in rapid flight [when the gods insisted she release Odysseus from her island: 'You are merciless, you gods, resentful beyond all other beings; you are jealous if without disguise a goddess makes a man her bedfellow, her beloved husband ... So it was when Demeter of the braided tresses followed her heart and lay in love with Iasion in the triple-furrowed field [the cutting of three furrows was part of the fertility rites inaugurating the agricultural year]; Zeus was aware of it soon enough and hurled the bright thunderbolt and killed him." -Odyssey 5.125

"Elektra, the daughter of Atlas, and Zeus were the parents of Iasion and Dardanos. Now Iasion had a lust for Demeter and was hit by a thunderbolt as he was about to attack her." -Apollodorus 3.138

"Iasion and Dardanos, two brothers [sons of Elektra], used to live in Samothrake. But when Iasion was struck by a thunderbolt because of his sin against Demeter, Dardanos sailed away from Samothrake, went and took up his abode at the foot of Mount Ida, calling the city Dardania." -Strabo 7 Fr 49

"There were born in that land [of Samothrake] to Zeus and Elektra, who was one of the Atlantides, Dardanos and Iasion and Harmonia … Zeus desired that the other of his two sons [Iasion] might also attain honour, and so he instructed him in the initiatory rites of the mysteries [of Samothrake], which had existed on the island since ancient times but was at that time, so to speak, put in his hands; it is not lawful, however, for any but the initiated to hear about the mysteries. And Iasion is reputed to have been the first to initiate strangers into them and by this means to bring the initiatory rite to high esteem.
After this Kadmos, the son of Agenor, came in the course of his quest for Europe [his sister abducted by Zeus] to the Samothrakians, and after participating in the initiation [into the mysteries of Samothrake] he married Harmonia, who was the sister of Iasion and not, as the Greeks recount in their mythologies, the daughter of Ares [actually the usual account was that Harmonia was given to Elektra mother of Iasion to raise as her own].
This wedding of Kadmos and Harmonia was the first, we are told, for which the gods provided the marriage-feast, and Demeter, becoming enamoured of Iasion, presented him with the fruit of the corn, Hermes gave a lyre, Athene the renowned necklace and a robe and a flute, and Elektra the sacred rites of the Great Mother of the Gods [Rhea], as she is called, together with cymbals and kettledrums and the instruments of the ritual; and Apollon played upon the lure and the Mousai upon their flutes, and the rest of the gods spoke them fair and gave the pair their aid in the celebration of the weding. After this Kadmos, they say, in accordance with the oracle he had received, founded Thebes in Boiotia, while Iasion married Kybele [here identified with Demeter instead of Rhea] and begat Korybas [leader of Kybele's Korybantes]. And after Iasion had been removed into the circle of the gods, Dardanos and Kybele [Demeter] and Korybas conveyed to Asia the sacred rites of the Mother of the Gods and removed with them to Phrygia ...
To Iasion and Demter, according to the story the myths relate, was born Ploutos (Wealth), but the reference is, as a matter of fact, to the wealth of the corn, which was presented to Iasion because of Demeter’s association with him at the time of the wedding of Harmonia.
Now the details of the initiatory rite are guarded among the matters not to be divulged and are communicated to the initiates alone; but the fame has travelled wide of how these gods [the Kabeiroi] appear to mankind and bring unexpected aid to those initiates of their who call upon them in the midst of perils. The claim is also made that men who have taken part in the mysteries become both more pious and more just and better in every respect than they were before. And this is the reason, we are told, why the most famous both of the ancient heroes and of the demi-gods were eagerly desirous to taking part in the initiatory rite; and in fact Jason and the Dioskouroi, and Herakles and Orpheus as well, after their initiation attained success in all the campaigns they undertook, because these gods appeared to them." –Diodorus Siculus 5.48.2

"Teams [of horses] which destroyed their drivers ... Horses destroyed Iasion, son of Jove [Zeus] by Electra, daughter of Atlas." -Hyginus Fabulae 250

"Those who were most handsome. Iasion, son of Ilithius, whom Ceres [Demeter] is said to have loved [Ilithios may have been the Pleiad Elektra's mortal husband & so Iasion's step-father]." -Hyginus Fabulae 270

"[Constellation Waggoner] Hermippus, who wrote about the stars, says that Ceres [Demeter] lay with Iasion, son of Thuscus. Many agree with Homer that for his he was struck with a thunderbolt. From them, as Petellides, Cretan writer of histories, shows, two sons were born, Philomelus and Plutus." –Hyginus Astronomica 2.4

"[Constellation Gemini, the Twins] Others have called them Triptolemus, whom we mentioned before [as the constellation Serpent-Holder], and Iasion, beloved of Ceres [Demeter] - both carried to the stars." -Hyginus Astronomica 2.22

"A rumbling argument arose in heaven, the gods all grumbling why others should not be able to grant such gifts [the restoration of youth to the elderly, the sole prerogative of the goddess Hebe]. Aurora [Eos] grumbled at her husband’s [Tithonos’] age, and gentle Ceres [Demeter] that Iasion was going grey." –Metamorphoses 9.421