Prince Phobos took the throne of the kingdom of Metamoor (known as Meridian in the TV series) some time prior to the opening chapter of the first story arc/season, and began to drain the life force of the people in order to fuel his own magical powers with the aim of becoming strong enough to take over other worlds. However, realizing that Phobos posed a great threat, the Council of Kandrakar, a realm at the center of the universe, raised a magical shield, known as the Veil around Metamoor, sealing Phobos inside and cutting it off from the rest of from all other worlds.
During the first story arc/season Phobos' primary concern is finding his lost sister Elyon Brown, whose powers are equal to his own (In the comic book, according to both Phobos and the Oracle, Elyon is even stronger than Phobos himself) and was spirited away to Earth approximately 12 years prior, so that he can drain her life force and add it to his own. With her power, Phobos would be able to break through the Veil and begin his conquest of other worlds.
At first, Phobos is unaware of his sister's identity, and sends his minions to find her, first by searching through birth records, and then using a magical talisman known as the Star of Therebe. After finding her and successfully poisoning her mind against her friends and family, Phobos brings his sister to Metamoor and attempts to gain her trust for long enough to drain her of her powers. Despite successfully draining some of his sister's power, Phobos' plans are foiled by the Guardians during the final installment of the Metamoor story arc/Season one. He is exposed as a villain, and is imprisoned by Elyon after a heated magical duel.
Following his defeat in Season 1 of the animated series, Phobos was briefly released by the Knights of Vengeance early in season 2, but was defeated while attempting to retake his castle from Elyon. He returned again late in Season 2, when the Guardians realized that, as an heir to the throne of Meridian, he was able to break the rule that "A Heart cannot be taken by force", and release him in order to deprive Nerissa of her Seal (which incorporated the Heart of Meridian), on the condition that he surrender the Seal to them afterwards. However, Phobos breaks his vow and instead uses the Seal to reconquer Meridian. After retaking Meridian, Phobos sets his sights on Kandrakar, and is on the verge of taking it when he is betrayed by Lord Cedric, who swallows him whole, thus gaining his powers for his own. After Cedric's defeat, Phobos is returned to Elyon's dungeon.
In the comic story arc "Trial of the Oracle", he became the new Oracle, as he was found to have used his negative emotions to switch minds with Endarno, who was the guardian of the Tower of Mists, the tower in Kandrakar where Phobos, Cedric, Frost, and many other villains were imprisoned in. Once he is restored to his rightful body by the Guardians, he realizes he has lost and leaps off of a balcony into the neverending void beneath Kandrakar, declaring that he will "never be imprisoned again".