Over 900 years ago, the followers of the first Azar were led to a place between all dimensions where they founded the Temple Azarath. It was Azar's plan to expel all evil from the Azarathians so that the demon Trigon couldn't use the dark energy to be fully re-established, and then Azar could finally kill him once and for all.
The dimension was destroyed by Trigon's forces when he finally got control of his daughter, Raven. However, all the inhabitants' souls survived, but were corrupted by the temporary merger with and destruction of Trigon. However, after the events of the Titans Hunt storyline, they were purified and guided into another plane of existence by the third Azar. The souls that remained eventually became the second incarnation of Phantasm.
One Year Later
In other media
In the cartoon series (as well as Teen Titans Go!), the character Raven invokes the name of Azarath in her chants to summon and focus her powers ("Azarath, Metrion, Zinthos!", to be exact). She left this pacifist society in another dimension to try to leave her demon heritage behind and become good by joining the Teen Titans, but she fears that her inbred evil is lying just under the surface.
In the episode "Switched" of Season One, Raven tells Starfire that she was born in a place called Azarath, but Raven does not specify that she, in fact, lived there. And during Season Four of Teen Titans, Raven visits a place she refers to as Azarath in the episode, "The Prophecy," to seek help from the spirit of her mother, Arella. However, the place is deserted and is later revealed to be an illusion left behind after Trigon supposedly destroyed it, which possibly implies that the "visit" Raven took was a spiritual or telepathic one, though no confirmation exists.