Blue Flame (青の焔) is a special story included in the DanMachi BD Volume 5 booklet.
As a child, Welf hated parties due to the insults that he would always receive. The party that he was currently attending was no exception, a boy around his age from a powerful family insulting him for being a Crozzo. He especially didn't like parties as none of it was true. Even if people acted friendly toward one another, on the inside they were always trying to gain the upper hand over others. However, Marius came to his aid, chastising the boy that was insulting him. Even though he was currently only twelve years old, Marius was already. The boy was startled for a moment but then began explaining to Marius that it was all Welf's fault for coming to the party, even going as far to mention that it was Welf that picked a fight with them without realizing his place. Welf ignored his insults though he couldn't control himself and punched the boy when he insulted metalworking by calling it "playing with metal". The boy's two friends joined the fight while Marius did nothing to stop Welf as he was too busy trying to hide his laughter.
The next day, Welf was in the back garden with Phobos, who was having a fit of laughter after hearing what he did the night before. Welf's mother was busy looking for him within the mansion so he hid out here to avoid a lecture. She remarked that it would be far more interesting if the Crozzo Family was made up of people like Welf. Phobos was the Goddess of her own Familia that was absorbed by Rakia following their defeat. Rakia contained a massive amount of soldiers which made it impossible for Ares to take care of all of them himself. Welf asked if Phobos thought about doing a coup against Ares but she was too lazy to train soldiers while evading Ares' watch. On top of that, all Level 2 and 3 soldiers were directly under Ares' control, and if any person was skilled enough, they would immediately be inducted into the Ares Familia. Phobos then remembered that Garon mentioned they would be forging that day, causing Welf to run off in search of them.
Although the forge was old like the mansion, Welf liked it there. As he greeted Garon and Vil by calling them old man and dad, Vil scolded him, wondering when he would start acting like a noble, continuing on to lecturing him about his fight the night before. Welf tried to protest but his father wouldn't have any of it, pointing out that it was only because of Marius that he got out of any trouble. Before his father could say anything else, Garon silenced him. Welf watched his grandfather smith for a while until he told Welf to try it. He was elated as this was the first time he had ever been able to start smithing. While he smithed, he never thought that his dream of making a weapon better than the Crozzo magic sword with his grandfather and father would never happen.
The fateful day came on Welf's tenth birthday. On that day, Welf received the falna from Phobos. Garon had made him wait until his tenth birthday so that he could learn the effort it took to smith before that. Once she finished, Phobos and the other Crozzos stared at the skill Crozzo Blood listed on his back. Getting an idea, Phobos told Welf to make a magic sword. He pointed out that he couldn't make one since the Crozzos were cursed but nonetheless he still made one.
Once the magic sword was finished, Welf and the others went out to the grasslands to test it. Everyone was amazed at its power which completely burned the grass around them. While the others were elated, Welf himself was saddened by the result.
Once they returned to their mansion, Welf's family surrounded him, telling him to make the Crozzo magic sword to restore their honor. Welf was opposed to it and refused to make it, causing his father to punch him. When Welf asked him about their dream to make a weapon better than the Crozzo magic sword, he told him to stop talking nonsense and called him a fool. He tried to gain support from the silent Garon though Garon told him to make the magic sword. That day, Welf completely cut any ties with the rest of his family.
That night, as Welf was preparing to escape, Phobos visited his room. Phobos apologized for the whole situation and offered to help him escape Rakia. She told him to take the second magic sword he created, the one that was going to be offered to the king. Welf agreed to her help and they decided to escape the next day.
At the throne room, Ares told Martinus that someone that could make the Crozzo magic sword had appeared. Martinus wasn't fully convinced of its effective, warning Ares that it might break just like when the Spirits first cursed them. Marius watched them from the side and asked the spy for more information about it. Upon hearing the name, Marius remembered him from the party a year ago. However, he still ordered for knights to be positioned at the wall.
The following night, Welf commenced with his escape. Valua had a total of four walls that separated the royalty and nobility, the military, the people, and the outside. Welf had easily passed the first two walls thanks to Phobos though he had been spotted at the third one. At the fourth wall, he saw that the gate was closed and knights were guarding it. He knew he had no chance if he attacked head on. Welf used the magic sword he had, completely destroying the gate and seriously wounding the knights. As he passed through the destroyed gate, he questioned creating magic swords.
Welf reached a wooded area not far from Valua. Once he arrived, Phobos appeared from within. She updated his status and also modified it so that he could convert to any Familia that he wanted to. She pointed out that her blood would still be there, which she equated to having taken his first time, causing Welf to tell her not to say weird things. Phobos informed him that she would take the blame for the whole thing and knowing Ares, he would believe it as he was an idiot. She was also going to do it for Welf and the fact that she was getting tired of working for Ares. Phobos said her goodbyes to Welf and left him alone.
Days later, a pillar of light shone from the direction of Valua. Welf sat on a hill and cried as he watched it.
A while later, Hephaestus visited the Sword Smithing City of Zolingham. While there, she noticed Welf fighting with other smiths for a spot. The head apologized for showing her the sight but she told him that smiths were like that. She then asked for information about him with the head telling her that Welf came a while ago, wanting to work. Hephaestus waited until Welf finished smithing a sword before she approached him. She asked for his name, then invited him to join the Hephaestus Familia. However, Welf wanted Hephaestus to name herself before she tried to invite him. Smiling, Hephaestus named herself.