"It's okay, Natsu." Runes flare to life at her hands. She can hear the others screaming behind her, but it's all dim, distant. END stands before her, growling at the low edge of human hearing, taut as a bowstring. The growling gets louder as Lucy's magic saturates the air around them, but he doesn't move. "It's okay. I won't let you hurt anyone else. And I won't let you go alone."
The spell finishes building, and Lucy takes just a second to hold it, to burn that last moment into her memory before she lets it loose. END staring at her, snarling, a nightmare of her Natsu; her friends screaming and sobbing at her back, her name fractured in a half- dozen agonized voices. The heat emanating from him, thick with malice, making it hard to think, to breathe, to move.
And in that split second, it's Mama she thinks of, Mama and her theory that the One Magic is love.
Let it be. she prays. She thinks of him, the one who brought her to Fairy Tail, brought her home. The boy who took her hand and offered her a fairy tale, a new adventure every day, Natsu with his beaming grin and bright eyes and steadfast heart. Everything that she knows of him, everything she feels for him, all the ways he's helped her grow- Lucy takes them and fills her heart and her magic with them, and her spell explodes into being.
END brought hellfire. Lucy Heartfilia brings hell itself.