Dahak, the One Evil God (previously called the One God or The One) was the impulse of destruction, a powerful, ancient evil being and a recurring character on both Xena: Warrior Princess and Hercules: The Legendary Journeys. Although his true form has only been seen once or twice on-screen, he possessed Iolaus' dead body. He impregnated Gabrielle with Hope, his one daughter and representative on Earth. He was one of the main causes of the Rift and the events that later led to Gabrielle's apparent death.
Whilst in Brittania, Gabrielle grew close to a follower of Dahak, Krafstar. He spoke about Dahak in way that Xena and Gabrielle thought he referred to the one God of the Israelites. Gabrielle soon discovers that this God and his followers are part of a sinister and dark cult, devoted to unleashing evil onto the Earth. Gabrielle is tricked into killing a member of the cult and then impregnated with Hope, the bringer of Evil.
Dahak then attempts to release his army of evil onto the Earth, through Hope and Ares. He is then slowed down by Xena, who manages to kill the Destroyer, his grand-child, who kills Hope with it. Dahak may be the most powerful and dangerous of all the antagonists of both series. 
Dahak was perhaps a Primordial entity that predates the existence of mortals, Gods, and even Titans. He was the embodiment of destruction or the "Impulse to Destroy", which is one of two of the most basic elements of existence, with the other being creation, or the "Impulse to Create". Dahak's greatest goal was to irreversibly taint the balance between creation and destruction, thus dooming the world into never-ending chaos. He once attempted to achieve this by entering the world and usurping the reign of the Titans during their war against the Olympians. However, the Titans were successful in repelling the evil God, but at the expense of the strength needed to defend against the Olympians, who took advantage of their weakened state and imprisoned the Titans in the Underworld.
Over millennia, Dahak has made an untold number of attempts to enter the world, which would include through the manipulation of mortals. One notable example was Zarathustra, a native from the lands of Sumeria, who was enthralled by the voice and promises of Dahak to the point where Zarathustra had even considered sacrificing his own wife and children in Dahak's name. However, when Zarathustra could not bring himself to harm his family and continue on the path of evil, Dahak punished him by killing his family and cursed Zarathustra with immortality so that the latter will never be free from the memories of his deeds leading to his family's death.
Since Dahak was the personification of evil and destruction, his weakness was the presence of anything that benefits good and creation. One example was the mysterious Stone of Creation, that could generate a divine light that was capable of rendering Dahak powerless.
While Xena and Gabrielle are traveling they come across some guards taking some prisoners to Britannia for trial. As they passed them Xena heard one of the prisoners mentions that they will never give in to Caesar. Upon hearing Caesar's name Xena battles the guards and releases the prisoners. One of the prisoners named Krafstar, informs Xena that Caesar has captured their temple, as well as they have hired Boadicea, Xena's former ally, to help defeat Caesar.
Xena and Gabrielle travel to Britannia with Krafstar to help Boadicea. Gabrielle talks to Krafstar about his "one God" and seems to make a friend in him. Krafstar uses his friendship with Gabrielle to trick her into killing Meridian, as well as therefore, losing her blood innocence, which is what Dahak needs to be able to enter the physical realm. Xena sees strange dark clouds above the temple and fearing for Gabrielle's safety, she rushes to the temple. When Xena enters the temple she finds it deserted, with a dead woman laying on the altar. She sees Gabrielle crying in the corner and asks her who had done this and Gabrielle tells her that she had. Krafstar enters the temple, as well as him and Xena fight. The altar bursts open and flames grab Gabrielle, as well as dangles her in mid-air. Krafstar transforms into "The Deliverer". Xena throws Krafstar into the flames and catches Gabrielle before she falls into the pit. Xena and Gabrielle take safety before the temple explodes. Dahak managed to impregnate Gabrielle before Krafstar's defeat.
Gabrielle carried Dahak's spawn for two weeks before giving birth to a daughter. Even though she is the seed of evil, Gabrielle tells Xena that because she is a part of her and that there must be some good in her as well: naming her Hope. Xena feared that Hope would be used as a pawn for Dahak, so Xena decides to kill her. Gabrielle, fearing safety for her child, sent Hope downstream in a basket, in secrecy from Xena.
Eventually Xena and Gabrielle cross paths again with Hope, though due to her supernatural genealogy, she's already grown into a pre-teen. Hope's manipulations cause Xena and Gabrielle to turn against each other, culminating in the death of Xena's son, Solan. Gabrielle eventually realizes Hope's true intentions and poisons her. The Dahak arc, however, continues over the Season 3 finale to an early Season 4 episode where Hope finally meets her demise. He planned on using Ares and Hope to sire a race of destroyers who would destroy the Earth and usher in his coming. However, both Hope and her child eventually meet their demise.
For a long while, Dahak battled against the Gods of Sumeria who were able to keep him at bay. The battle caused chaos on Earth and King Gilgamesh used his palace to shelter his people. Gilgamesh blamed his father, Ra, for allowing his wife and child to be killed and secretly became a follower of Dahak. In order to weaken the Gods, Gilgamesh drank the nectar that sustained them from the Chalice of the Gods. The gods were then weakened enough to allow Dahak to enter the world. The last step was the sacrifice of a "Warrior Heart." Gilgamesh intended to sacrifice his half-sister, Nebula, but Iolaus saved her life by sacrificing himself. In turn, Hercules killed Gilgamesh by throwing him into the fires of Dahak (HTLJ "Faith").
Dahak lured Iolaus' spirit into the realm where Dahak is still a prisoner and disguise his environment as the Elysian Fields. Taking on the form of Hercules, Dahak offered Iolaus "the power of a thousand Gods" in order to liberate the Earth from the Olympian Gods' reign of terror. Iolaus had for a while refused Dahak's offer, but eventually gave into temptation by taking Dahak's hand. Dahak had finally entered the Earth in Iolaus' body with Iolaus' soul as a prisoner. The balance of light and darkness began to shift in Dahak's favor through every tragic event: The Sumerian Gods have died as a result of having nothing to sustain themselves and Dahak himself had killed the noble druids of Ireland, who acted as mentors to Hercules after Iolaus' apparent death. Dahak had mentally tormented Nebula forcing Hercules and his new companion, Morrigan to return to Sumeria and confront Dahak now in possession of Iolaus' body. Dahak made a vague attempt to make Hercules believe that he is indeed Iolaus just with the power to achieve whatever he wants. Hercules, however refused to believe that he is Iolaus. Dahak easily escaped Hercules' grasp and journeyed to Greece. Hercules, accompanied by Nebula and Morrigan headed to Greece as well, to stop Dahak or die trying.
When Dahak arrived at Greece, he found that the Olympian Gods, who were severely weakened by his dark presence, had already left before his arrival. Dahak sensed that his former ally Ares had not left with them, instead choosing to hide. In spite of this, Dahak chose to adopt a disguise as Iolaus the new "God of Light" in order to gain followers and trust of the people who believed that the Olympian Gods had abandoned them. He even made them believe that Hercules is the reason for the Gods departure, claiming he has returned to destroy them. Hercules wanted to know what Dahak had to gain from his deception and Dahak states the wants a return to the old days before the Olympians ruled the earth. Dahak was said to have been defeated by the Titans when he first manifested on Earth. This battle was said by Mnemosyne to have devastated the planet and left the Titans so weakened that Zeus was able to defeat them and claim mastery over the Earth. The extensive devastation also resulted in the Greek Gods rebuilding the planet into something more hospitable. Mnemosyne advised Hercules to gain the "stone of creation" in which its purest light will render Dahak powerless. After these revelations, Hercules gains a new ally, Zarathustra, who was once a follower of Dahak a thousand years prior to one of Dahak's attempts to enter the Earth. Zarathustra reveals that there are two basic forces that govern the universe: creation and destruction and that Dahak is the force of destruction. Zarathustra reveals to Hercules that Dahak wants Hercules to kill him in order to condemn Iolaus' soul to eternal doom, giving Dahak the means to plunge the planet into chaos. After obtaining the stone of creation, Hercules and Zarathustra confronted Dahak and bind him in chains using the stone to prevent Dahak's escape. Zarathustra begins a ritual to exorcise Dahak's being from Iolaus' body. Dahak taunts Zarathustra for killing his family and cursed him with immortality a thousand years ago in order to feed off of Zarathustra's anger and gain enough strength to kill him, preventing the ritual from continuing. Dahak revealed to Hercules of how he manipulated Iolaus into accepting him into his heart.
During this time, Dahak taunts Hercules further by telling him that Iolaus was nothing more than a travelling companion. Hercules tells Dahak that his friendship with Iolaus is one of the best things that he has been blessed with. This moral sentiment upsets Dahak and weakens him enough for Iolaus to call out to Hercules from the spiritual realm Dahak has him trapped in. Hercules touches Iolaus' hand and is sucked into the spiritual realm where he sees his long dead friend. Dahak appears there too, in his true form: A huge crustacean-like monster. Hercules and Iolaus battle Dahak and send him into a fiery pit. With Dahak defeated and the balance of creation and destruction had returned to normal, Dahak's spirit was finally liberated from Iolaus' body. Zarathustra returns and guides Iolaus into an after life governed by the force of creation, which is later hinted at being the Judeo-Christian-Islamic God.
Powers & Abilities
Dahak has the same Powers and Abilities of a God, as the embodiment of darkness/evil, is one of the most powerful, his mere presence weakened and affected the Olympian Gods so they had to leave Greece and when Ares choose to stay, his Godhood was weakened, until Dahak was defeated.
Dahak is equally or more powerful than Zeus himself and rivaled all Titans in power, and in the first great war that happened all the Titans had to combine his efforts to defeat him. By the time they defeated him, the Titans were so weak to defend themselves against the Olympians, so the standouts powers of Dahak are:
Weakening divine power: Dahak is more powerful than all Earthly Gods, has the power to weaken, all kinds of divine powers of the Gods, his presence weakened the entire Olympian Pantheon, who had to flee another dimension and did not have enough power to face him. When Ares lodged in Greece his powers were weakened.
Power to kill the Gods: Dahak has the power to kill a god, as demonstrated by giving his blood to Loki in the First / Alternative Future that Hercules changed and his blood killed in that future Balder, the Norse God of the Light.
It seems that Dahak has the ability to reincarnate on Earth, which does not completely die. In antiquity was defeated by the Titans and thousands of years later, he possessed the body of Iolaus.
- Aži Dahāka or Dahāg is a three-headed dragon in Persian mythology. It appears in the Avesta, the earliest texts of Zoroastrianism. Zarathustra's presence strongly suggests this as his influence. Interestingly however, the supreme evil of Zoroastrianism is not Aži Dahāka, but Angra Mainyu (or Ahriman).
- There are some contradictions within the series regarding Dahak. The chief druid tells Hercules that he is a creature from beyond this universe implying he is from a different reality or dimension. Yet Dahak's recounting as well as that of the Titans suggests that he is a manifestation of the darkness within the universe, which would indicate he is native to this universe, and the counterpart to the One God.
- It is unclear whether Dahak died or was imprisoned within his own realm once more by Hercules and Iolaus.
- While Dahak (being the embodiment of the will to destroy) is the archenemy of the Elijan God (the embodiment of the will to create), he is not the show's representation of the Devil. Michael would later allude to Mephistopheles being the angel who rebelled against the order of the Archangels and becoming King of Hell. This would indicate that the Elijan God was constantly at war with both Dahak and Mephistopheles at the same time.
- It is unlikely that Dahak would have a counterpart in the Strange World, as he is described as an inter-dimensional being by Mabon; this would infer that there is only one Dahak, and he is spread between every universe.
- Dahak implied to Hercules that he was the serpent in the mythological Garden of Eden.
Xena: Warrior Princess
Hercules: The Legendary Journeys
- "Norse By Norsevest"
- "Somewhere Over the Rainbow Bridge"
- "Darkness Rising"
- "Let There Be Light"