Magus is an antagonist in El Goonish Shive. He's the "orchestator" of the Sister arc. He comes from another universe, where a woman named Terra defeated him in a battle and he got stuck in the main universe on an astral plane. Some people, like Sirleck and Chaos, can still see him.
In the Sister arc, he influenced Tedd so that he would accidentally turn Elliot into a girl. His plan was to impregnate female Elliot at some point, but this failed when Tedd and Elliot found the Dewitchery Diamond, splitting female Elliot into Elliot and Ellen.
His next plan was to make Ellen to turn Elliot female again by her body-altering beams, but it failed when Sarah (Elliot's love interest) snapped her out of trying (and failing) to be a villain, leading her to try to get a normal life as Elliot's sister. Magus is now waiting to repeat his plan by... stalking Ellen and Elliot.