In the story about the Gigantes or giants, we must distinguish the early legends from the later ones. According to Homer, they were a gigantic and savage race of men, governed by Eurymedon, and dwelling in the distant west, in the island of Thrinacia; but they were extirpated by Eurymedon on account of their insolence towards the gods.
Alcyoneus: shot by Hercules.
Clytius: burnt by Hecate with torches.
Enceladus: fled but Athena threw the island of Sicily on him.
Ephialtes: shot by Apollo with an arrow in his left eye and by Hercules in his right.
Eurytus: killed by Dionysus with a thyrsus.
Mimas: killed by Hephaestus with missiles of red-hot metal.
Pallas: flayed by Athena who used his skin to shield her own body in the fight
Polybotes: chased through the sea by Poseidon threw the island of Nisyrum on him.
Porphyrion: Zeus smote him with a thunderbolt and Hercules shot him dead with an arrow
The other giants Zeus smote and destroyed with thunderbolts and all of them Hercules shot with arrows as they were dying.
From The Theogony of Apollodorus
Homer accordingly looked upon the Gigantes, like the Phaeacians, Cyclopes, and Laestrygones, as a race of Autochthones, whom, with the exception of the Phaeacians, the gods destroyed for their overbearing insolence, but neither he nor Hesiod knows any thing about the contest of the gods with the Gigantes.
Hesiod (Theogony 185), however, considers them as divine beings, who sprang from the blood that fell from Uranus upon the earth, so that Ge was their mother. Later poets and mythographers frequently confound them with the Titans, and Hyginus calls them the sons of Ge (Terra) and Tartarus. Their battle with Zeus and the Olympian gods seems to be only an imitation of the revolt of the Titans against Uranus.
Ge, it is said (Apollodorus i), indignant at the fate of her former children, the Titans, gave birth to the Gigantes, that is, monstrous and unconquerable giants, with fearful countenances and the tails of dragons.
They were born, according to some, in Phlegrae (i. e. burning fields), in Sicily, Campania, or Arcadia, and, according to others, in the Thracian Pallene. It is worthy of remark that Homer, as well as later writers, places the Gigantes in volcanic districts, and most authorities in the western parts of Europe.
In their native land they made an attack upon heaven, being armed with huge rocks and the trunks of trees. (Metamorphoses by Ovid V)
Porphyrion and Alcyoneus distinguished themselves above their brethren. The latter of them, who had carried off the oxen of Helios from Erytheia, was immortal so long as he fought in his native land; and the gods were informed that they should not be able to kill one giant unless they were assisted by some mortal in their fight against the monsters.
Ge, on hearing of this, discovered a herb which would save the giants from being killed by mortal hands but Zeus forbade Helios and Eos to shine, took himself the herb, and invited Heracles to give his assistance against the giants.
Heracles, indeed, killed Alcyoneus, but as the giant fell on the ground, he came to life again. On the advice of Athena, Heracles dragged him away from his native land, and thus slew him effectually.
Porphyrion attacked Heracles and Hera, but was killed by the combined efforts of Zeus and Heracles, the one using a flash of lightning and the other his arrows.
The other giants, whose number, according to Hyginus, amounted to twenty-four, were then killed one after another by the gods and Heracles, and some of them were buried by their conquerors under (volcanic) islands.
The fight of the giants with the gods was represented by Phidias on the inside of the shield of his statue of Athena.
The origin of the story of the Gigantes must probably be sought for in similar physical phenomena in nature, especially volcanic ones, from which arose the stories about the Cyclopes.