Erza and Gray Join Team Natsu

Erza and Gray join Team Natsu is an event during the Lullaby arc where Erza Scarlet is assisted by Natsu Dragneel, Gray Fullbuster and Lucy Heartfilia in her goal to stop the dark guild Eisenwald from completing their evil scheme.

Prologue
After returning from their last mission, Natsu and Lucy lounge in the Fairy Tail Guild. Later on, they are informed that Erza Scarlet has returned to Fairy Tail. The armored Mage, known as Fairy Tail's strongest woman, soon enters and begins berating the guild members for the trouble they've caused. After Erza calls out to them, a fighting Gray and Natsu buddy up and tell Erza they've been hanging out, much to Lucy's surprise. Erza then reveals that she has an urgent mission she must deal with and requests Natsu and Gray to accompany her, much to their surprise.

Event
At Magnolia Station, Gray and Natsu argue about having to go together on the mission, until Erza arrives, stopping them. Erza welcomes Lucy, who was asked to go by Mirajane, into the team. Before leaving, Natsu challenges her to a fight after the end of their mission, which she accepts. On the train, Erza knocks out a motion-sick Natsu and explains that their mission is to stop the dark guild known as Eisenwald from getting their hands on an artifact known as the Lullaby.

Upon exiting the train, the three Mages realize that Natsu is not with them, meaning he is still on the train. While on the train, Natsu is confronted by a member of Eisenwald known as Kageyama who begins insulting Fairy Tail for being legally recognized. The dark Mage picks a fight with Natsu and initially hits him due to Natsu's motion sickness. However, when the train suddenly stops due to interference from the team elsewhere, Natsu is able to fight back and knock the Mage away, causing him to run off swearing that Natsu would regret his actions.

Upon jumping out of the train, Natsu is caught by the rest of the team on a Magic Four Wheeled Vehicle. When Natsu reveals that he fought a mage from Eisenwald, Erza immediately berates him for letting him go despite the fact that their mission was to stop them, though Natsu tells her he didn't know, having been unconscious while she explained the mission.

When Natsu mentions that the Eisenwald mage carried a strange flute with a three-eyed skull, Lucy recalls a story about The Magic of Death; Lullaby, a curse which killed anyone who heard its fatal song. Knowing what could happen if Erigor of Eisenwald got his hands on the Lullaby, the team races off to stop the dark guild.

Aftermath
At Kunnugi Station, Eisenwald takes over the train and forces all passengers off. Meanwhile, Kageyama meets with Erigor and presents him with the flute, claiming to have undone the seal on it by himself. Erigor revels in how the flute was once just a tool for "murder by curse" at first, but after entering the dark Mage Zeref's hands, it was further developed it into a Magical flute, called "mass-curse murder magic" which can kill anyone who hears it.

The dark guild quickly takes over Oshibana Station and defeats the Rune Knights who attempted to stop them. At the same time, Team Natsu appears at the station and quickly confronts the dark mages.