I Surrender My Power Over To You

When Laxus protects Lucy from some kind of shadow that attacks her, he loses his left arm in the processes. During the attack, Laxus chooses Lucy as his Mate, and he gives her half of his magic power, this makes Lucy alot stronger. But what are these Shadow things attacking and why do they want Lucy? Can Laxus protect her? Why does Natsu want Lucy away from Laxus? Lemons!