Mara, often short for Mara-Papiyas, is the lord of the sixth heaven of the Desire Realm and the head of gods in Mount Meru. It is said that if a man could reach beyond the sixth heaven, he will then overcome all kinds of desires and delusions to go on the path to enlightenment. So there is only a line between the Buddha and the devil!
But why would the demon lord summon him? Yeah bcz demon lord was bored wants to fight against a new toy or he was fed up of his immortality and wants to die now.