At the end of the Suthorn Ruins, you are attacked by the Seismic Talus. This page explains how to defeat it.

How to Defeat the Seismic Talus

First Phase

Seismic Talus will pull back one of its arms, then throw the arm at the wall, so make sure to move out of the way so you don't get hit. When arm will break when it hits the wall, and blue energy will come out. Collect the blue energy.

Sometimes, Seismic Talus will raise both hands, which will glow purple. Then it will slam its hands down on the ground, so stay far away until Seismic Talus's hands are stuck in the ground. Press Up on the directional pad when the hands are in the air, to switch to swordfighter form. Then when the Talus's hands are stuck in the ground, quickly run behind the Seismic Talus, and swing your sword repeatedly to hit the glowing orb at the base of the Talus. Or, if you run out of Swordfighter energy, run behind the Talus, hold ZL to target the orb, and press X to bind to the orb, then pull on it.

After you hit it twice, or pull on it with bind, the orb will fall off, and blue energy will come out. Hit the orb with the sword until it disappears (press Up when it disappears, to conserve your swordfighter energy), and the Seismic Talus comes back up from the ground. This time, the glowing orb will be in Seismic Talus's shoulder.

Second Phase

At this point, Seismic Talus will sometimes stick its arms out, and they will glow purple, and the hands will spin in a large circle, so stay far away.

Wait for the Seismic Talus to raise both of its hands, then press Up on the directional pad, and when the hands get stuck in the ground, hit the glowing orb twice with the sword, or hold ZL to target the orb and press X to bind, then pull on it, to make it fall off and drop blue energy. Keep hitting it until it disappears. Switch back to normal form.

Third Phase

Seismic Talus will now turn red, and the glowing orb will be on top of its head. Its hands will glow red, so move away, and it will get its hands stuck in the ground. Switch to swordfighter mode, stand on the hands, and jump up, then swing your sword to hit the orb, or if you run out of swordfighter energy, hold ZL to target it, and press X to bind to the orb (you might need to jump on the hands to make sure Tri can reach the orb), then pull out the orb. Then hit it repeatedly with your sword. Repeat this strategy until the battle ends.

After the Battle

Watch the cutscene. Tri will level up, which allows you to have four active triangles at a time. (Each echo costs a certain number of triangles, which are displayed above the echoes in your notebook in the + pause menu.)

Tri gives you 5 Unusual Crystals.

Your swordfighter energy is refilled.

Pick up the Heart Container to add a heart to your health meter (or leave it there if you want an extra challenge).

Then go to the glowing spot and talk to Tri to exit the dungeon.

There will be a cutscene. Go south to follow the person you met. When you arrive at the destination, there will be a cutscene. A couple of large rifts will be marked on your map.

This completes the quest "The Mysterious Rifts" and starts the quest Searching for Everyone.