The "Rune Symbols" that spreads all across Elan's bare body act as codes to channel dark energies to cast various powerful magic. One of the most impressive aspects is the ability to open warp gates that connects to demonic plane, summoning and bringing powerful demons to presence of human world.
Elan's sister, Lune (who is the main female character of the story) also has Runes marked on her right hand, as a present given by her brother before he left to travel. Through this, Lune can also call upon demons to presence, but with the cost of her brother's life force.
Once opens the gate, Lune must call the name of the demon she wishes to summon, and the phrase with that name for it to be effectively brought to this world. As the demon stays longer in here, the taxing of her brother's life will be greater to sustain the power needed maintaining the demonic presence.
However, as bound to the rules of Heaven and Hell, the summoned demon will fulfil its bidding in fasted and most effective way as it can.