“I suppose I should thank you for barging in here. If you hadn't gone to the trouble of destroying my life's work, I would feel a slight pang of guilt at making you suffer!”
- ― Arkarium, talking to the player right before his boss fight
The details below contain SPOILERS, you may want to view this after completing the storyline in-game.
Prior to the events of the game, Arkarium was a servant under the Goddess Rhinne in the Temple of Time, until he presumably betrayed her to join the Black Mage's forces. While he was still loyal to Rhinne, he was sent to the kingdom of Kritias to find a source of immortality for King Hekaton.
Upon learning that the Demon gave Mastema a letter to deliver to the Heroes, Arkarium attacks her/him and leaves her/him for death. She/He transforms into a cat in order to survive, looking for a way to revert her/his form and regain her/his powers.
When the Demon tries to confront the Black Mage, Arkarium stops him. After a scuffle, Arkarium attempts to use an all-out attack until the Black Mage's doors open, inviting the Demon to his throne, and Arkarium leaves.
During the Silent Crusade Storyline, Arkarium created several Mystic Gates and empowered many Master Bosses (ie- Timer, Zeno, etc). He finally entered the Leafre of Past through a crack in the Temple of Time and attempted to unseal his master. He was then stopped by a Mapler, although it turns out that this Arkarium was a decoy and the real one vanished to the Dimensional Schism. In an expedition, he was finally defeated once and for all.
Arkarium makes two cameo appearances in the Black Heaven storyline. He is also is one of several possible bosses who may participate in leading the assault in the Kritias invasion (if so, the Temple of Time mobs are included in the army).
In FriendStory, Arkarium's counterpart is the elderly science teacher at Shinsoo International School. While not a bad man, the repressed jealousy he feels at the affection Hilla and Magnus feel towards each other (or so it seems) makes him easy prey for an Envy-fueled Troublemaker, whom the player faces at the conclusion of Chapter 4.
In Heroes of Maple, he commences Damien to take on his mission on the World Tree. However, Arkarium double crossed Damien, revealing that he had manipulated him from the very start. This results in Damien using the power of the Transcendence Stone to increase his power and attacks him, to the point that Arkarium has to retain in his snake form.
He is also responsible for the events of Twisted Aqua Road, having corrupted the royal family of the long lost kingdom. His insignia, a menacing eye, is seen embedded on Antoinette's forehead. It is unknown what his goal is at the Twisted Aqua Road.
Later on, the player encounters Arkarium once again in Morass, attempting to trick Tana into helping the Black Mage. Tana, having lost her memories and possessing power beyond what is known, turns the entirety of the Morass into a replica of Kritias, before it was struck by disaster. As a result, the player and the Flying Fish are also affected, being turned into Shey, a traveling priest, and Jean, a local boy, respectively, and are forced to relive the memory of Tana.
In the 52nd year of the Hekatonian calendar, a tragedy struck Trueffet Square in Kritias; a rope supporting an Anti-Magic Stone snapped, causing the stone to fall and explode, killing everyone in the vicinity except for one girl: Tana, whose body regenerated in front of everyone, but causing her to lose her memories. She was then captured by the guards and taken to the dungeon of Trueffet, where she was experimented on to find the source of her immortality for King Hekaton. Having experimented on her for nearly 2 years, the researchers discover that while her body regenerates itself perfectly, her memories and emotions are always wiped, leaving Tana as a blank slate. Due to the nature of the experiments being both inhumane and beyond their understanding, the former High Priest resigned, stating that "This is neither magic nor science. It is beyond human; it is of the gods. We have overstepped our bounds."
However, a new High Priest has arrived to assist in the experiments and to help Hekaton ascend to immortality, none other than Arkarium (who was still serving the Goddess Rhinne at the time). Arkarium made great progress, but struggled to find the reason for the recipients rejecting the transfer of Tana's power (leading to their bodies disintegrating, but their souls remaining, desperately attempting to recreate bodies in the form of horrible monstrosities), and wanted to ensure that Hekaton would not encounter the same fate. During another experiment, King Hekaton appeared, demanding the researchers increase the power of their experiments beyond the limit. A massive explosion occurred as a result, but Hekaton was unharmed (aside from a strange aura surrounding his body, due to him slowly losing his humanity).
Jean breaks into the dungeon to visit Tana repeatedly, comforting her and reading stories to her, including a story with a knight rescuing a princess. One day, Tana reacts and is about to explode, but Jean manages to calm her down by hugging her, ending the reaction, and leading Arkarium, who witnessed this unbeknownst to Jean, to realize that Jean is the key to ensuring a successful transfer of power to King Hekaton. Jean formulates a plan to escape with Tana, and asks for Shey to help him find an Anti-magic Stone Necklace as a wedding gift for Tana. Jean and Shey then encounter Arkarium, who tricks the two into thinking he wants to help Tana escape to prevent a horrible disaster from engulfing Kritias, as well as causing Tana endless pain.
On the day of Jean's plan, Shey (who regains consciousness as the player) realizes that Arkarium, who was never truly under the influence of the Morass, led the two of them into a trap, and Arkarium calls for the guards (which he originally brainwashed the real Shey to do), who then proceed to kill Jean, leading to the ritual succeeding, and proceeding with the transfer of power to King Hekaton. However, Jean had a backup plan: the necklace he gave to Tana contained unstable magic, which she took advantage of by destroying mid-ritual, leading to her body disappearing and Hekaton being cursed eternally.
When the player asks Arkarium why he caused these events to reoccur in the Morass, Arkarium replies by saying that while he expected the ritual to fail, he was curious as to how. He then kidnaps Tana and attempts to transfer her power to himself, having learned that Tana has the power to control Erda, the source of all life. The player goes to take down Arkarium after realizing that Jean, who was really the Flying Fish, is still alive. After the player fails to defeat Arkarium, Jean appears to console Tana once more, ending the ritual. Tana, who has now gained control of her powers, defeats Arkarium.Arkarium, in a severely weakened state, goes to the Black Mage of the past to ask for his powers to be restored. However, the Black Mage of the past gives him one final message...
I am not the one you seek. The Black Mage commanded me to relay this message to you...
You have done well. Rest.
...and promptly kills Arkarium.
While Arkarium does not directly make an appearance in Tenebris, he is briefly mentioned in a conversation between Neinheart and Grendel the Really Old where they were discussing how everything went according to the Black Mage's plans. Accordingly, the Black Mage deliberately gave the Demon credit in capturing Rhinne which would cause Arkarium to be jealous, which would lead to Demon shattering the Black Mages defenses and leading to his ensealment. Arkarium's jealousy also involved Damien's recruitment and him absorbing the World Tree's powers which would scatter her powers across Maple World. All of this was apart of the Black Mage's plan.