Tartarus is a prison located in the Underworld.
The prison; Tartarus, was built by Hephaestus. It was designed with the concept of being impenetrable, and to maintain such a concept, Hephaestus built numerous failsafe mechanisms, including the labyrinth. Tartarus was built to hold the titans, specifically Kronos. It is compromised of many stairways and ruined temples, all of which connect to an enormous diamond shaped structure resembling two fused pyramids, which presumably holds the Titans. The Titan King himself lies encrusted between two mountains, at the bottom of a dimly lit valley. Lying before Kronos is a stairway which ends in a precipice. At the end of the stairway, there appears to be the remains of a temple, which serves as an altar. Zeus is placed here by Hades in order to transer the divine powers of Zeus to the onlooking Titan. This process sees a river of magma spill over the edge of the precipice from Zeus' arms, which then engulfs Kronos.
Labyrinth: A massive constantly morphing labyrinth from which no one is known to have escaped before. It is a dark and eerie place, not for the faint hearted. In the final section of the labyrinth is located the Minotaur, to prevent anyone from entering or exiting the labyrinth. Interestingly, when Perseus defeats the Minotaur, the labyrinth shifts and reveals the stairway to Kronos.
After Zeus was taken prisoner, he was brought before Kronos. Ares binds Zeus between two pillars, and when Zeus seemingly taunted his son, Ares began to beat him in a fit of anger. Hades was forced to step in and knock Ares down, less he should beat his father to death. Ares protested that Hades should not be merciful, but Hades retorted that Ares is too bloodthirsty. Kronos slowly began to drain Zeus' energy.
When they are seen again, the magma has become a river, which has spilled over the precipice towards Kronos. Ares has been sent off while Hades keeps watch over Zeus. The Gods argue again, with Zeus questioning the logic behind siding with Kronos. Hades replied that he has no choice, seeing as he will be spared from a terrible fate. Hades then explains that when a God dies, their souls vanish from existence. Hades wishes to avoid this.
They are not seen again until Perseus, Andromeda and Agenor reach the end of the Labyrinth. They immediately head to the alter where Zeus is trapped. While this is happening, Zeus calls out to Hades again. Zeus then gives an apology to his brother. Hades is surprised, and asked why he is apologizing. Zeus answered that he regretted banishing him, and encouraged his brother to redeem himself. Hades is visibly conflicted, but Ares sees this and knocked Hades aside, believing that Hades has no pride. Hades was shaken, but seemingly unharmed. Why Ares attacked is unclear, although he seemed to believe that Hades was still loyal to his brother.
However his attempt was thwarted when Zeus refused to allow the thunderbolt to activate in his presence. Ares was slightly annoyed, but the three Gods were interrupted by Kronos. As the Titan suddenly awoke, Hades decided to act quickly. In retaliation Hades swung the pitchfork into Ares' head, stunning his nephew. As Ares staggered, Hades then used the pitchfork to catch Ares' arm. Ares parried it aside and tried to swing his hammer, but Hades ducked down. Hades then charged into Ares with enough force to throw him over the precipice. The two of them fell from a great height, and Hades disappeared from sight while Ares landed on a lower cliff. Both of them survived the fall with little to no injuries.
Shortly afterwards, Perseus, Andromeda and Agenor arrived to free Zeus. Perseus used Poseidon's trident to try and break Zeus' bonds. While they struggled, Kronos began to free himself and attempted to kill Zeus. Andromeda tried to break the chains with her sword. At last Perseus freed his father. Ares then attacked the group, easily defeating Agenor and overpowering Andromeda. He was about to finish them until Hades pinned Ares down by the arm.
This allowed Zeus and the others to escape, just as Kronos' hand destroyed the altar, But as Hades and Ares struggled, Hades attempted to stab his nephew. Ares narrowly avoided this and made a grab for the pitchfork. Hades managed to restrain Ares, but Ares threw the pitchfork in a last ditch effort to kill Zeus. The pitchfork impaled Zeus in the back, wounding him. Kronos then realised what Hades had done and was seemingly angered. For some reason Ares chose to abandon his uncle, possibly to pursue Zeus, but Zeus combined the pitchfork and the trident to transport them out of the underworld. Ares then found the dagger made by Helius, which Perseus had dropped. Presumably Hades followed them, as he is not seen in the Underworld again with Ares.