Apart from Al'taieu being the zone traversed in order to reach the Limbus zones of Temenos and Apollyon, these zones also serve as the locations of the powerful notorious monsters known as Jailers, each of which is capable of dropping some of the best accessories in the game.
In order to gain access to Al'Taieu, one must complete Chains of Promathia Mission 7-5 A Warrior's Path. While there's only a minimum level of 30 needed in order to start the CoP missions, it is decidedly difficult to attempt any of the missions below the designated level cap.
Empyreal Paradox (Quest/Mission Battlefield)
In order to spawn Jailers, particular items obtained from other Jailers or from non-Jailer notorious monsters.