Tephra Cave

The Tephra Cave is a cavernous area in Xenoblade Chronicles. The cave itself is located inside the Bionis' right leg. It must be navigated at least two time throughout the game, the first time in order to reach the Cylinder Hangar and the second time to access Gaur Plain. At first it can be reached via the landmark named Tephra Cave Entrance at Colony 9.

Story
This cave is the main path from Colony 9 to the upper regions of the Bionis such as Colony 6. A permanent squad of the Colony 9 Defence Force is within the complex.

Reyn was tasked with gathering Cylinders to operate a Mobile Artillery that was crashed into a house by soldiers from the Defence Force in the Residential District of Colony 9. The actual Cylinder Hangar appears to be part of a larger ship, which Shulk theorizes was made by ancient Homs.

After that the Mechon launch an assault on Colony 9, killing many Homs in the process including Fiora, giving a motive to the group for traversing the caves in order to reach Colony 6 where they hope to find a way to destroy Metal Face, the Mechon who led the attack. While in the cave, they reach some dead travelers. They decide to allow them to "return to the Bionis", leaving the corpse in a body of water. After this, they decide to rest. After Shulk wakes up, they are attacked by a group of Arachno, which take Reyn hostage. Shulk tries to catch up with him, scared because of a vision where he saw Reyn being killed by an Arachno Queen. After a long chase, he manages to catch up with Reyn just before he is killed by the said Arachno. However Shulk manages to unlock Monado Shield and saves Reyn. Together they manage to kill this beast and safely exit the caves, but not before theorizing that the spider was the reason why nobody from Colony 6 had reached Colony 9 for a while.

After the events in Mechonis Core, a new area of Tephra Cave opens up, which contains enemies of level 90 and up. The new path can be found at Vilia Lake, where there were previously rocks blocking the path. The new area also contains one of the Giant ruins.

Normal Enemies

 * Air Vang
 * Arachno Pod (normal)
 * Arm Bunnit
 * Cave Skeeter
 * Clap Bunniv
 * Cute Caterpile
 * Desert Krabble
 * Director Arachno (normal)
 * Eater Caterpile
 * Iron Bunniv
 * Jewel Krabble
 * Medium Skeeter
 * Mell Lizard
 * Night Wisp
 * Noble Brog
 * Officer Arachno
 * Plane Bunnit
 * Prom Skeeter
 * Singing Brog
 * Tephra Lizard
 * Weather Flier
 * Willow Bunniv
 * Wind Vang
 * Worker Arachno (normal)

Post Mechonis Core

 * Brabilam Vang
 * Dorsiar Lizard
 * Falsel Bunniv
 * Fascia Bunniv
 * Femuny Wisp
 * Hanz Pod
 * Jada Arachno
 * Jadals Arachno
 * Panasowa Arachno
 * Rhoen Quadwing
 * Sonicia Gogol
 * Vilae Arachno
 * Young Brog

Quest Exclusive Enemies

 * Devoted Arachno
 * Dim Wisp
 * Greedy Arachno
 * Lurker Brog
 * Lurker Krabble
 * Resolute Arachno
 * Zealous Arachno

Post Mechonis Core

 * Satisfied Gogol

Story Exclusive Enemies

 * Arachno Pod (story)
 * Captain Arachno
 * Director Arachno (story)
 * Soldier Arachno
 * Worker Arachno (story)

Unique Monsters

 * Cellar Bugworm
 * Gluttonous Eugen
 * Mining Patrichev
 * Solid Konev
 * Wallslide Gwynry

Post Mechonis Core

 * Erratic Goliante
 * Judicious Bunnitzol
 * Musical Vanflare
 * Plump Sprahda
 * Protective Torquidon
 * Reckless Galdon

Quest Exclusive Unique Monsters

 * Dazzling Tolosnia
 * Firework Geldesia
 * Reckless Zanden

Boss

 * Arachno Queen

Heart-to-Hearts
{| class="wikitable sortable" !Name !Requirement
 * The Legend of the Spider
 * Shulk and Reyn - Cloudy Affinity
 * A Scene Revisited
 * Reyn and Fiora - Heart affinity
 * Glowing in the Night
 * Dunban and Riki - Cloudy Affinity
 * Glowing in the Night
 * Dunban and Riki - Cloudy Affinity
 * Dunban and Riki - Cloudy Affinity

Trivia

 * Tephra Cave is the first area where Ether Crystals can be mined.
 * }