Arcanos is the first and greatest Magico, and the source of all other Magicos.
Backstory
It is still unknown where, whence, and how exactly Arcanos came to be. It is an entity unlike any other currently known, spanning the size of a dwarf galaxy. Its true nature is something like a parasitic universe or plane unto itself.
Arcanos seeks only to feed and expand. It sends out its minion magicos to devour, and invariably end, all life in their path. There can be no negotiation with the magicos, being that they are all chained to the will and hunger of their progenitor. It is a state of existence that some of the smarter ones such as the Magico Lord Garak detest.
Like an Eldritch horror from Lovecraft, Arcanos travels the space between universes, snuffing out all life in its wake. As great and terrible as it is though, the speed and spread of the menace of magicos is still ultimately limited. New universes are born in the multiverse at a rate that greatly outpaces the speed at which Arcanos is able to move and eat. While this means that the vast majority of universes across the multiverse will never have to deal with Arcanos and its ilk, this is actually worse news for those universes it does come across. Arcanos' limited threat on a multiversal scale means that the forces of Heaven and Hell also consider it beneath their greater notice, thus leaving it to the local inhabitants of those universes to deal with on their own.
One of the first known encounters with Arcanos within the local universe was an attack launched by Marianne. As a first contact, the exact nature of Arcanos was unknown then, but an army of Magical Girls was deployed in the hopes they could end the magico threat once and for all. Only two survived. These days, the Anti-Magico Alliance do their best to try and keep Arcanos in check, and to kill its minions wherever they may be found. There is no light at the end of the tunnel to be found however, as there are currently no known ways to deal with Arcanos permanently.
Personality
For those first confronted with the threat of the magicos, they may imagine some great evil mastermind behind it all. They would be wrong. Arcanos itself is in fact not even a fully sapient, thinking entity; it is barely even sentient in some ways.
Arcanos has no true goals or greater thoughts; the only thing that drives it is the need to expand. It has been likened to something akin to an insect colony, almost like its greater encompassing actions are like an emergent property of a collection of smaller actions. Its thought processes, if they can even be called that, are lesser to and more primitive than even most basic animals. It can also be thought of as being something like a force of nature in some ways rather than an independent entity that makes its own choices. Lastly, the way it invades new universes, infects them, uses them to create more magicos, and ultimately leaves them dead while seeking new universes in which to repeat the process endlessly is remarkably like a virus; perhaps the most apt comparison of all.
It tends to be depressing for the prey that Arcanos feeds on when they learn the truth of its nature it; to know that their extermination serves no greater purpose. They were merely in the way of something more powerful than them, and it thought and felt nothing about their destruction.