Tethe'alla (テセアラ?), the "Prosperous World" (繁栄世界?, "Prosperity World"), is the second explored world in Tales of Symphonia. It is part of the cross-dimensional world Aselia, mirrored by another world, Sylvarant. The flow of mana is controlled between the two worlds, and during the events of the story, Tethe'alla is the world receiving the mana flow, resulting in its cities being significantly more advanced in magitechnology. The world of Tethe'alla is fairly different than the world of Sylvarant, as Tethe'alla has a king who rules over the land, elves are still alive, and half-elves are at the bottom of the caste system. Additionally, some Tethe'allans still have the ability to summon, an art that has been lost in Sylvarant. Less importantly, even the music in Tethe'alla differs from its counterpart, being more rocky and adventurous. Tethe'alla's moon is named "Sylvarant", and "Tethe'alla" is the name of Sylvarant's moon. In Tales of Phantasia, Aselia has two moons, both of which are named after the dimensions that were fused to create it.
Tethe'alla is comprised of four continents, which are not specifically named. Rather, they are distinguished by their most noteworthy settlements. The planet's largest continent, located in its southwestern hemisphere, houses the capital of the world, Meltokio. Just south of the city lie the Fooji Mountains, after which the continent is sometimes named, and the southern tip of the continent is home to the Temple of Darkness, where the Summon Spirit of Darkness, Shadow, resides. The continent also owns a House of Guidance, which is a hermitage chain analogous to Sylvarant's House of Salvations.
The Temple of Earth, where the Summon Spirit of Earth, Gnome, resides, sits at the northern tip of the continent, which connects to a peninsula housing the Hot Spring area. To the north of the peninsula rests a small chain of islands. The Fooji continent connects to Tethe'alla's middle continent with the Grand Tethe'alla Bridge, powered by 3,000 Exspheres. The middle continent is home to the university-centered Sybak, as well as the hidden village of Mizuho and the forested Ozette. Near Ozette lies Altessa's House, which is only slightly northeast of Tethe'alla's Tower of Salvation. Gaoracchia Forest leads to the northern tip of the continent, where a nearby chain of islands house the Temple of Lightning, where the Summon Spirit of Lightning, Volt, resides.
While Tethe'alla's large, southeastern continent houses the hidden elf village of Heimdall in the Ymir Forest, the region is more notable for the resort city Altamira, which is nestled at the edge of a large island north of the continent. The majority of the continent is empty, with no other major settlements to be found. There is, however, a House of Guidance and South East Abbey, which is similar in function. Noteworthy areas include Latheon Gorge, Toize Valley Mine, and the Otherworldly Gate, which is located at the far, northeastern tip of the continent. The ruins are rumored to be a portal to Sylvarant. In Tethe'alla's northeastern hemisphere, the small, ice-coated Flanoir continent consists of only the snowy city itself, the Renegades' Tethe'alla Base, and the Temple of Ice, where the Summon Spirit of Ice, Celsius, sleeps. The continent has numerous glaciers surrounding it, making sea travel difficult, so air transportation is generally preferred.
Tethe'alla was once a country part of the world known as Aselia. Around 5,000 years prior to the events of Tales of Symphonia, a conflict known as the Kharlan War ignited between Tethe'alla and another country, Sylvarant, due to the advent of magitechnology. The war lasted for at least 1,000 years, when finally, a young boy who would later become known as Mithos the Hero successfully forged pacts with the world's Summon Spirits and ended the war with his three allies. The war, however, caused irreversibly damage to the planet's World Tree, the source of the planet's mana, so Mithos used the Eternal Sword to manipulate space-time and split the planet of Aselia into the separate planets of Sylvarant and Tethe'alla.
Due to the mass depletion of mana, the two worlds become forced to vie for mana in a seesaw-oriented system of mana links. Mithos constructed a system in which the hourglass planets house an even number of Summon Spirits to correlate yet oppose the other planet's Spirits in an elemental fashion. Through this system, neither planet was able to advance too greatly in technology before succumbing to the opposing planet's accumulation of mana. This aids in deterring the reinvention of magitechnology and a return of war. In an effort to manage the balance of mana between the two worlds, a Chosen of the lesser planet embarks on the Journey of World Regeneration to unlock the seals at a series of elemental temples. The unlocking of these seals leads to the reversal of the mana links, which in turn, redistributes the flow of mana into the declining world from the more prosperous world, thereby leading to its decline while the lesser world's civilization is given the opportunity to progress forward.
The process is repeated when the flourishing planet grows too advanced technologically, and the opposing world's Chosen ventures on the regeneration journey to reverse the mana links once again. Around 800 years prior to the events of Tales of Symphonia, the Chosen Spiritua successfully managed to reverse the mana flow in favor of Sylvarant, but multiple failed Chosen following Spiritua's success eventually led to Tethe'alla flourishing for a lengthy period of time. During the story, Tethe'alla thrives in magitechnology but has resorted to enforcing a strict caste system that places half-elves on the lowest level.