Tethe'alla 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 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 Spirit 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 Tetha'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 Spirit 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 Spirit Celsius sleeps. The continent has numerous glaciers surrounding it, making sea travel difficult, so air transportation is generally preferred.
- Melktokio - Known as the Imperial City, Meltokio is the largest city in Tethe'alla and serves as its capital. The city is divided into four districts: the Slums, where the poorest citizens live; the Commoner's Quarter, where middle-class citizens reside; the Middle District, where armor shops, the central square, and the world-famous coliseum are located; and the Upper District, housing the nobles' quarter, Martel cathedral, and royal palace. The city also has a large sewer system that can be used as an alternate access to the city.
- Sybak - Known as the University Town, Sybak is well-known for its Imperial Research Academy and great scientists. The city's library contains a number of rare, historical texts that provide the group with information on Colette Brunel's Chronic Angelus Crystallus Inofficium and later the Kharlan War.
- Mizuho - Known as the Mystical Village, Mizuho is a hidden village populated by ninjas that carries a heavy Japanese theme. The village is located near the Tower of Salvation and surrounded by mountains, the Gaoracchia Forest, and a lake. The village chief is Igaguri, who has been in a coma for several years, leading the vice chief Tiga to perform his duties in the meantime. The villagers' named are not their true names since only respected families of a person are granted this knowledge. The names of Sheena Fujibayashi, Kuchinawa, and Orochi are aliases used as public identities. The village has a small islet, known as the Isle of Decision, where duels take place.
- Ozette - Known as the Quiescent Village, Ozette is a forested village on Tethe'alla's central continent. The village resembles a giant tree, which lies in a part of the forest where Sacred Wood grows, and it is infamous for its contempt of half-elves. During the story, the village is destroyed by Cruxis for hiding the nearby dwarf Altessa. The village is also the hometown of Presea Combatir and the Pope. After the world regeneration, Presea returns to rebuild Ozette. The group does not visit Ozette in Tales of Symphonia: Dawn of the New World, but they learn from Presea that reconstruction is moving slowly, and that the village has a reputation of opposing the Church of Martel, which is hampering the reconstruction efforts.
- Flanoir - Known as the Snowy City, Flanoir is the large and only settlement of Tethe'alla's northeastern continent. The constant snow in the town is attributed to the Spirit Celsius, who resides in the nearby Temple of Ice. The city has an excellent doctor who treats both citizens and people all over the world, albeit for a high price.
- Altamira - Known as the Seaside Paradise, Altamira is a major city in Tethe'alla. It is a seaside resort popular for housing the Lezareno Company headquarters, a hotel resort, a large beach, a shopping center, and a theme park. The theme park's mascot is the Klonoa.
- Heimdall - Known as the Hidden Elven Village, Heimdall is the legendary home of the elves, hidden deep within the Ymir Forest. Half-elves are singularly banned from entering the village's gates due to acts committed by a half-elf to the elven race hundreds of years prior to the events of the story. Stories celebrating Mithos as a hero are also banned within the village, and the Storyteller was consequently forced to relocate to Latheon Gorge because of this. The few half-elves born within the village are allowed to stay as long as they never leave or cause problems.
- Exire - Known as the City in the Sky, Exire is a floating city that soars across the skies of Tethe'alla. It is home to many half-elves who live there to escape discrimination and persecution from the world beneath them. Many of the half-elves living in the city were banned from Heimdall. Raine Sage's mother, Virginia Sage, lives alone in a small house near the edge of the city. Exire is considered an ancient city, having been lifted off the ground some 4,000 years ago by the Spirit Maxwell as part of his pact with Mithos. The city's architecture is dated, and its buildings and monuments are falling apart. The city is divided into platforms supported by Maxwell's power that are connected by narrow, grassy walkways.
- Fooji Mountains - Located on the Fooji continent, from which the mountain chain obtained its name. The mountains form a range on the continent, peaking at high altitudes and containing very little greenery—with the exception of a few trees.
- Gaoracchia Forest - A maze-like forest that separates Meltokio from Ozette and Mizuho. Within the forest are light-sensitive plants that react to the Sorcerer's Ring once it gains the power of light. It is where the powerful Sword Dancer can be fought for the second time in the game, as well as where Sheena's grandfather found her when she was young.
- Toize Valley Mine - A mining cavern owned by the Lezarano Company. Its primary function was to excavate Exspheres from the earth and power Tethe'alla's technological advancements. It is the primary reason behind the company's rise to economic power. "Inhibitor Ore", which is needed to create Key Crests, can be found in the mine, albeit behind a large, high-tech security system.
- Tethe'alla Base - Located within the Flanoir continent's surrounding glaciers, the base serves as the Renegades' Tethe'alla stronghold.
- Ymir Forest - A pond and bridge area that surrounds the city of elves, Heimdall.
- Latheon Gorge - Home of the "Mana Leaf Herb", as well as the Storyteller who was banished from Heimdall.
- Tower of Salvation - A large tower that appears in Tethe'alla due to the planet currently flourishing with mana. It is the same place as Sylvarant's Tower of Salvation.
- Torent Forest - A serene, woodland area located within Heimdall. The Spirit Origin resides in the forest.
- Temple of Lightning - A tower housing the Summon Spirit of Lightning, Volt.
- Temple of Earth - A cave housing the Summon Spirit of Earth, Gnome.
- Temple of Ice - A cave housing the Summon Spirit of Ice, Celsius.
- Temple of Darkness - A fortress housing the Summon Spirit of Darkness, Shadow.
- Grand Tethe'alla Bridge - A large drawbridge that crosses the stretch of water between the Fooji continent and the Sybakian continent. Its mechanisms are powered by 3,000 Exspheres.
- Altessa's House - Where the dwarf Altessa lives with this robot doll, Tabatha.
- House of Guidance - Similar in function to Sylvarant's House of Salvations. These hermitages are typically occupied by clerics.
- South East Abbey - A House of Guidance isolated from civilization to protect Seles Wilder, Zelos Wilder's half-sister.
- Dragon's Nest - A large, floating nest that roosts several young dragons.
- Otherworldly Gate - A mysterious formation of rocks that, according to legend, is a portal to Sylvarant on nights of a full moon.
- Hot Spring - A bath area located on a Fooji continent peninsula.
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.
- The name Mizuho is based on one of Japan's more ancient and poetic names. Likewise, the culture of the village, architecture of its buildings, and clothes and names of the villagers draw influence from the Sengoku period.
- In Norse mythology, Heimdallr is a god who possesses the resounding horn Gjallarhorn, owns the golden-maned horse Gulltoppr, and is the son of Nine Mothers.
- Exire is a Latin verb meaning "to exit, depart, or go out", symbolizing the half-elves' departure from the world below.
- In Norse mythology, Ymir is a primeval being born of primordial elemental poison and the ancestor of all jotnar.