Thoon sided with the giants in the First Giant War. In the myth, Thoon and Agrius were clubbed to death by the Fates with maces of bronze.
Thoon was first introduced in Piper McLean's dream by Enceladus, in response to Periboia's remarks about tempting fate. Enceladus replied that Thoon was born to kill the Three Fates, saying, "He will strangle the old ladies with his bare hands. He will shred their yarn and destroy their loom. He will destroy Fate itself!".
Later, Thoon was called upon by Porphyrion to sacrifice the captured Percy and Annabeth for Gaea. Before he could perform the deed, however, his hand was chopped off by Piper. He later faced off against Percy Jackson, somehow having managed to reattach his hand.
Thoon was bludgeoned to death by the Fates in the final battle of the Gigantomachy, after which Hades (invisible with his Helm of Darkness) opened up an abyss under the fallen giant's body, sending Thoon back to Tartarus.
- Prowess in Battle: Despite being a rather short giant, Thoon is a very skilled warrior (fighting with a huge meat cleaver), able to hold his own against Percy Jackson himself, though he is later defeated through the combined effort of all three Fates.
- Giant Invincibility: As a giant, Thoon can only be killed by a god and demigod working together, and when he is harmed by anybody else, he heals at an accelerated rate. Hence, he was brought down the first time around by Hercules and the three Fates, and the second time around in The Blood of Olympus by the three Fates and some of the Seven.
- Destiny: As the anti-Fates he has the ability to control Destiny, the opposite of Fate. This is his strongest ability.
Thoon is described as a shriveled, wizened giant, with ratty grey hair, a wrinkled face, eyes milky with cataracts and dragon-scale legs as white as frost. Instead of armor, Thoon wears a tattered sackcloth tunic. He is apparently smaller than most of the Giants, including Enceladus (who is thirty feet tall).