|“||I devour magic. I destroy the voice and the soul. You cannot oppose me.||”|
During the first Gigantomachy, Clytius was created to absorb all of Hecate's magical abilities. However, Hercules, Dionysus, and Hecate worked together to defeat him, with Hecate lighting his hair on fire as the finishing blow.
When Leo Valdez and Hazel Levesque were cut off from their friends in the House of Hades, they battled Clytius and Pasiphaë. After the witch was defeated, the rest of the Seven and Nico di Angelo fought Clytius until Hecate came and, with the help of Hazel, killed him.
According to Leo Valdez's dream, Clytius wears a robe made of shadows and is twenty feet tall. Clytius' dragon-like legs are the color of ash and his braided hair is described like his shadow robe in Hazel Levesque's account of his appearance. Due to his black shadow robe, usually only his bright eyes are visible. With the shadowy cloak dissipated, Clytius's complexion is as dark as Thanatos', with his blue eyes "glinting cold as diamonds". His massive humanoid upper body was encased in black Stygian iron armor, and he had a huge black Stygian iron sword, five times the size of Nico's.
Clytius, as the bane and opposite of Hecate, has a number of powerful and unique abilities though the extent of his power is still unknown :
- Prowess in Battle: Clytius has enhanced strength, and during the original Gigantomachy, was only brought down by Hercules, Dionysus, and Hecate working together. In The House of Hades, he was capable of holding his own against multiple opponents simultaneously, but was finally overwhelmed by most of the Seven battling him together, and killed by Hecate once again.
- Magical Absorption: As the bane of Hecate, Clytius has the ability to absorb nearly all of her magical attacks, along with Leo's fire, all of which simply passes through his black mist cloak, though the cloak itself is later dissipated by Nico, and the giant is still susceptible to most powerful fire-based attacks from Hecate. He claimed to be strong enough to erase the Mist implying tremendous power.
- Giant Invincibility: As a giant, Clytius 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. He was taken down the first time around by the combined efforts of Hercules, Dionysus, and Hecate, and the second time around in The House of Hades by the combined efforts of Jason, Frank, Nico, Leo, Piper, and Hecate.
- Dark Mist: Clytius exudes a dark mist which can cause demigods and mortals to faint upon contact. He can use the people whom have fainted due to this mist to speak, as he has no voice of his own. However, speaking through others saps the life force of the mortal or demigod, as Hazel saw happening to Percy and Annabeth
- At the end of The House of Hades, Clytius was one of the two giants that are currently dead, since all of his brothers, barring Damasen, have returned to the mortal world. As such, Clytius and Damasen are the only giants that do not re-appear in The Blood of Olympus.
- Gaea claims Clytius is her favorite son, though this actually being fact is debatable, as Gaea has been dishonest multiple times in the past.
- Clytius is the only Giant born to oppose a Titan. Similarly, Ephialtes and Otis were born to oppose Dionysus while he was still a demigod.