Arithas has spent most of his life in Augustana, in Andoran. His family owns an alchemist's shop there, and that is where he learned his trade. His father taught him to make various potions to cure numerous ailments. Arithas wanted more from alchemy though, and he soon discovered that he could create better things using his innate magical powers.
He learned how to make extracts, spells in the form of a bottle that he could use to enhance his abilities. From reading he discovered how to make bombs, explosive firey cocktails (losing his eyebrows a few times in the process).He even learned out to make mutagens that could change his form into something stronger, faster, or tougher. He quickly learned more than his father could dream of teaching him, making him independent and self-reliant.
One day, while he was demonstrating to his father the things that he had learned, a cry rose up from the city walls, "Yellow sails on the horizon!" Slavers from Okeno were headed to take vengeance on the pirate-hunter fleets of Augustana. While the city watched the naval battle from the land, one of the yellow-sailed galleys separated itself from the conflict. The ship moved closer to the bay, attacking the port with its ballistae. When the galley was close enough, they launched boats to come aboard.
With much of the city's protector's fighting in the bay, Arithas and many other of the city's men took it upon themselves to repel the invaders. Arithas fought using his new found abilities. He drank an extract that made him able to throw bombs even farther, and a mutagen that allowed him to throw them with greater accuracy. Soon, the slavers were retreating, but not without a few of the townspeople; among them a friend of Artithas.
When the battle was over a man approached him and told him the news of his friend. Arithas became enraged, and started for the water, threatening to swim to the galley to save his lost companion. The man stopped him and told him there were better ways to exact revenge than suicide. The man told him of the Pathfinder Society, and that the Andorans within the Society sought to bring freedom to all. The man told him that with training from the Society, he could save his friends and the townspeople later, though it would take time. Along the way, Arithas could master alchemy, a long-time goal of his.
With his father's blessing, and a hatred of slavery, he set out to serve Andoran by joining the Pathfinders with the purpose of freeing slaves around the world, and mastering alchemy in the process.
