Disclaimer: I don't own the Skull, Rogue, or Mage classes. This story is just my own little thought about how the Skulls came to be.
For thousands and thousands of years, thieves, rogues, and robbers have always infiltrated other demons' property for riches and power. But one day a young rogue was born, but he never wanted to steal. Then more refused, so they were banned from their clans to the harsh, cruel Netherworld.
To get back at their family and friends for expelling them from their only homes, the enraged rogues grouped together, learning magic spells from the Mages of the Forest, four mysterious mages that each know a different type of magic, and formed "the Skulls". The Skulls wore the traditional mask to disguise themselves as rabbit-like demons in the forest, and each different color mask represents each different element of magic. Red for fire, blue for ice, green for wind and brown for star.
The Skulls first grouped themselves in the outskirts of Resena, a roughhouse town near the forest filled with rogues and thieves, and the red Skulls initiated the first act by setting random houses on fire. Then the green Skulls spread the fires around onto the people with their wind magic. Then the blue Skulls freeze the people, and finally, the brown Skulls use their star magic to break the ice, thus destroying the people.
The Skulls had won that time, even though they didn't manage to destroy all the rogues. After the incident, some have killed themselves to atone for their sins. And from the generations after that, the Skulls have become more peaceful, but, like their ancestors, they wield powerful magic, as well as an uncontrollable anger. Now they are known as the Mage's male counterpart because of the fact that they know the same skills as the Mages.