-1I don't own Naruto
As Naruto stared, the sword-master tried to pick up the blade. A blast of lightning came from it, delivering a rather hefty shock. At Naruto's surprised look, he explained "Each of the swords has a defense mechanism to stop other people from using them." Naruto looked confused and asked why he could pick up Zabuza's sword after the battle. The man shrugged and said "Zabuza never did the ritual you did. He attacked the Mizukage and fled before the sword was finished." Naruto had no response to that and began to examine his sword more closely. The hilt had several silver and black strands wrapping around it for better grip. There was a v-shaped extension coming out of the hilt, with the blade coming from the open end of the V. As Naruto continued to stare, the sword-smith began speaking again "The color of the blade reflects the most powerful and common emotions you have. Black represents hatred, anger, sorrow, and pain. Of course, your element was lightning, and you don't have a favored animal." Briefly, Naruto wondered why there wasn't something about a fox on his sword, but he realized that since the Kyuubi and him were two separate spirits, the fox didn't have any influence on Raijin. He came back to reality when he realized the man was speaking again. "Now we have to find out what your sword can do. Pick it up, and try channeling your chakra through the blade."
Naruto quickly did as he was told and picked up the blade. He thought the sword was around eight pounds, light enough to be wielded easily in one hand, and heavy enough to do serious damage to his opponent if it connected. All of this ran through Naruto's mind as he brought the blade up in front of him and began pumping his chakra into the sword. Black colored lightning sparked and Naruto started to flow more chakra into the blade. Suddenly the whole blade section of the sword seemed to grow hazy and was crackling like mad. Naruto was so startled he dropped the sword and the blade immediately returned to its normal state. A disapproving glare later and the sword was back in his hand. He started flowing chakra through the blade again, this time steeling himself not to drop the sword. As the sword again started to crackle, Naruto put more chakra into the blade, bringing it back to the state it was before he dropped it. As he held the sword at arms length he noticed the blade had gotten lighter, but he didn't really acknowledge that since he was completely shocked by what he saw. The entire blade was now a bolt of black lightning. Crackling with electricity, Naruto thought the blade could cut through anything. He took a few experimental swings with the blade going through trees and even rocks like a hot knife through butter. After a couple of minutes the man who he simply called sensei called him over. "Alright, now that we've found the principal ability of your sword, I can start deciding which sword style to teach you. You are small, have a short reach, are not very flexible, are very strong physically, and have a sword that can cut through damn near anything. So you need a sword style that helps you use what you have to your best advantage. Since your sword almost can't be blocked, and adding your strength into the mix, it's pointless to have you go around your enemies' defenses, so I'm going to teach you a style very similar to the one Zabuza used. It focuses primarily on powerful slashes that smash through your opponents' defenses. I'm also going to be teaching you how to block jutsus with your sword."
By this point Naruto was salivating at the prospect of learning all of the stuff sensei was talking about. He quickly wished he hadn't been so enthusiastic. Sensei set up a brutal training regimen, doubling Naruto's weights, and ordering him to do one-thousand sit-ups, push-ups, pull-ups, and a strange push-up Naruto had never even heard of before: the diamond. Instead of having his arms beneath his shoulders, his thumbs and index fingers were formed in the shape of a diamond, and placed under his solar plexus. Naruto thought it would make more sense to put it under his nose, but after just a few of them he realized exactly how hard it was having your hands so far back. And that wasn't the end of it. Sensei brought out a sword so heavy Naruto could hardly lift it with both hands, and ordered him to do one thousand slashes left-to-right, right-to-left, upwards, and downwards. Naruto's favorite slash quickly became a move where he brought the sword behind his head, and brought it straight over and continued in a powerful downward slash. After that, Naruto had to work on blocking jutsus with his sword. While sensei was not a ninja, he had learned several techniques and could probably be ranked as an upper chunnin. "For smaller techniques like the 'Suiton: Teppodama', you bring your sword around and simply knock them away from you. For larger techniques like the 'Suiton: Suiryudan', you bring your sword in front of you and flow chakra through the blade to split the attack and let it go on either side of you. We will work on the 'Teppodama' until you can block them easily, then we will work on the 'Suiryudan'" With that sensei formed several seals and fired twelve water bullets from his mouth. Down, left, up, down, Raijin deflected the bullets, but five of them still got through. At the end of the day, Naruto was soaking, bruised all over, and felt like he'd been fed through a meat grinder. That was the way things went for the next four days, until Naruto was able to block all of the water bullets sensei sent flying at him.
His strength had increased, as he was now capable of lifting the weighted sword with one hand. He still looked rather clumsy wielding it with one hand but it was still a great improvement. Sensei now sent Naruto to work on the trying to block the 'Suiryudan'. The first time he sent the dragon at Naruto, the boy had forgotten to flow chakra into the blade. The dragon slammed into him and sent the boy flying. Several tries later he was starting to get the hang of it. "Suiton: Suiryudan no Jutsu." Success! The dragon hit Raijin and was split straight down the middle, leaving Naruto completely unharmed. Sensei nodded with a pleased expression on his face.
At the end of the training, Naruto set off to get back before he was late and Gai started to panic. He had never asked sensei's name and the man had never given it. Still, he felt a sort a familial attachment to the man. He had taken Naruto under his wing and treated Naruto like he felt a father would treat him. Kind but stern, encouraging him but not letting him slack off or do poorly. As he was musing on these things he walked into Tazuna's house with his sword strapped to his back, and Gai sporting a relieved expression, with a little worry and anger mixed in, met him. Sensing that whatever was coming was not going to be good, he hastened to reassure his sensei. "I met a man at Zabuza's grave. He said he was the maker of the swords of the Seven Swordsman of the Mist. When I told him about the battle he asked me if I would like him to make me a sword too. I accepted and he took me to his workshop and made me a sword." At this point Naruto showed Gai his sword and outlined his training. Gai was not to be so easily appeased. "Naruto, that was a very foolish thing to do. You had no idea if that man really was who he said he was. He could have killed you, or taken you for the Kyuubi, or any number of unpleasant possibilities you didn't even begin to consider. I'm very glad you're okay, but never do such a thing again. As punishment, when we return to Konoha, you will run five hundred laps around the village while wearing triple your weights." Naruto paled. That was a serious threat. Still, Naruto was glad he had went with the man. He had gotten a very cool sword, learned a powerful sword style, and had learned how to block jutsus with his new blade. All in all, he felt the reward was worth the punishment. He and Gai got up early the next day. Gai had finished the mission, and it was time to return to Konoha.