It could be rogue ninja from a hidden village. They might be after Tenten because she has some sort of valuable information or a powerful weapon in her possession. Naruto gets in their way to protect her, and these rogue ninja use their forbidden jutsus to overpower Naruto.
One possible plotline could be that Naruto and Tenten are on a mission together. Enemies attack and Naruto, being the selfless hero, puts himself in harm's way to protect Tenten. Maybe they are in a village that's being invaded, and Naruto uses his Rasengan to fend off multiple attackers while Tenten is injured or pinned down. In the end, Naruto sacrifices himself, leaving Tenten with a deep sense of loss and a newfound determination to carry on Naruto's will of protecting the village and its people.
In the fanfiction, Naruto might be targeted by a powerful enemy who wants to steal his Nine - Tails chakra. Tenten, being a skilled ninja, steps in to protect him. She could set up all kinds of traps around their hiding place using her weaponry skills, and engage in combat with the enemy when they get too close, buying time for Naruto to regain his strength or for backup to arrive.
Tenten would likely be deeply traumatized. Naruto was not only a comrade but also a symbol of hope and strength. His death in protecting her would make her feel guilty. She might start to question her own worthiness of being saved at the cost of Naruto's life.
One way this could unfold is that Naruto and Tenten are on a mission together. They are suddenly attacked by a powerful and unexpected enemy. Naruto, being the selfless hero he is, sees the enemy about to strike a fatal blow on Tenten. He jumps in front of her without hesitation, taking the full brunt of the attack. His last moments could be him telling Tenten to keep fighting and protect the village.
In the fanfiction, it could be that Gai has been poisoned or influenced by a dark force. His behavior becomes erratic and he targets Tenten for some unfounded reason. Naruto, being the hero he is, jumps in front of Tenten to shield her. He might then try to find a way to cure Gai while also keeping Tenten safe. He could call on his friends like Sakura for medical help and use his own ninja skills to fend off Gai's attacks.
Naruto might be really confused at first. He's used to people being friendly or having normal crushes, but a yandere is a whole different level. He'd probably try to talk to her and understand why she's acting so strangely.
Their relationship could start with mutual respect. Naruto respects Tenten's dedication to her weapon - based ninjutsu, and Tenten respects Naruto's strength and determination. They might start training together more often, which would lead to them spending a lot of time together. This time together would allow them to open up to each other about their pasts, their fears, and their hopes for the future. As they grow closer, they would start to develop feelings for each other, and these feelings would eventually lead to their marriage.
Maybe Rock Lee could be an important character. He could come to the orphanage to help with the physical training of the orphans. He would be a great inspiration for them with his hard - working attitude. Also, Neji could be involved. He could use his Byakugan to keep an eye on the orphanage and make sure there are no threats approaching.
In this fanfiction scenario, Naruto giving Tenten a sword could be a really interesting plot point. Maybe Naruto found a special sword during one of his adventures and thought Tenten, being a weapons expert, would appreciate it. It could lead to Tenten developing new fighting techniques with the sword, and perhaps she would use it in future battles against powerful enemies.