Mattias has medium-length brown hair and wears a blue coat with fur trim.
As a merchant, Mattias puts on a modest and kind personality. He is nevertheless eager to call others’ faith into question, believing that people suffer because the Sacred Flame turns its back on them.
Under the surface, Mattias possesses an incredibly twisted sense of self-righteousness, thinking himself the “one true Savior” destined to resurrect Galdera so that he might gain his power. Driven by his hatred of the Sacred Flame, he has wholly dedicated his life to that goal. He is skilled at finding a way into people’s hearts in order to bend them to his will, and will not hesitate to use lies, murder, and manipulation to accomplish this. He does not believe in love.
115 years prior to Octopath Traveler, Mattias, once called "Matthew," was a priest of the Sacred Flame in the Frostlands. The village was struck by lightning, starting a wildfire. Despite his fervent prayers to the Flame, countless lives were lost. This caused Matthew to question his faith and eventually reach a "revelation."
That same year, Matthew gained 100 years free of aging from Lyblac and established the Obsidians alongside Simeon. He sought to weaken the Sacred Flame and allow more of Galdera's power to exit the Gate of Finis, which would grant him more power.
Over time, he slowly cut his ties with the church and began using Galdera's power to bring about miracles. He established a cult following as the Savior of Wispermill in order to gain access to the altar in the Ebony Grotto. He worked his way back into the church under a new identity, as the trader Mattias.
In the final step of his plan, Mattias murders Archbishop Josef with poison provided by the Obsidians. Upon the Archbishop's death, he plots to take advantage of the next Flamebearer, Lianna, using the despair of her father's passing to manipulate her into acting as his vestal in order to bring Galdera's flame to Orsterra. He convinces her to drug her adoptive sister and acting Flamebearer, Ophilia, in order to steal Aelfric's Lanthorn.
While conducting the ceremony at the Ebony Grotto, Mattias instructs Lianna to pray to the Flame with her greatest desire. His followers begin to collapse, their purpose being sacrifices to the dark flame. He is soon confronted by Ophilia, who was discreetly freed by Lianna, and is defeated. Following his death, he is forced to exist in a dark limbo with no light.
A journal containing his thoughts after death can be found at the Gate of Finis. He cannot understand how Ophilia was able to stop him as he truly believed he was the destined savior, and despairs about being trapped in the void.
Mattias begins with 4 shields, but upon being broken, may increase them by two, for a total of 8 shields.
Mattias will summon help during battle, calling two Senior Cultists, two Black Matter, and two Augmented Remnants, in that order. His weaknesses will be locked while these enemies are alive. After Augmented Remnants are defeated, Mattias will no longer be able to summon help.
- Consuming Darkness: Dark elemental damage to a single target.
- Black Gale: Dark elemental damage to entire party.
- Dark Blessing: Elemental atk. and speed up to all enemies for 3 turns.
- Infernal Prayer: Physical atk., elemental atk., and speed up for Mattias.
- Mattias Summoned Help!: Used at 75%, 50%, 25% and 10% health. Summons two Senior Cultists, then two Black Matters then two Augmented Remnants to the battlefield respectively with each cast. After the first and third time summoning help, he will use Infernal Flame the next action. At 10% health Mattias will call for help a final time and the message "But nobody came..." will display.
- Infernal Flame: Applies an aura to the battlefield that makes your party members unable to use magical skills. Goes away if Mattias is broken.
- Mattias's Scepter Surges With Power: Used at 25% health. Uses Black Thunder next action if not broken.
- Black Thunder: Very heavy lightning elemental damage to entire party. Chance to inflict unconsciousness.
9,892 HP. Shield 7
- Eye Scratch: Physical attack, single target. Chance to inflict blindness.
- Fireball: Fire elemental damage to entire party.
- Fire Storm: Used at low health. Fire elemental damage to entire party. Strikes twice.
12,365 HP. Shield 5
- Shadow Strike: Dark elemental damage to a single target.
- Black Magic: Dark elemental damage to entire party.
- Dancing Shadows: Used at low health. Dark elemental damage to random targets.
14,838 HP. Shield 6
- Bite: Physical attack, single target.
- Rend: Physical attack, targets entire party.
- Rampage: Used at low health. Physical attack, targets random allies.
Saint Matthias was the apostle chosen to replace Judas Iscariot following his betrayal of Jesus and death.
- The boss battle with Mattias is one of only two to have in-battle dialogue during the battle, the other being Darius. However, unlike the latter, there is voice acting during the dialogue.