Sasuke's reaction could be complex. At first, he would be enraged. After all, the Rinnegan is a symbol of his power and his journey to gain strength. However, he might also have some mixed feelings. He and Naruto have a deep bond, and he might wonder if Naruto had a good reason for taking it. But overall, his initial reaction would likely be one of fury and a desire to get his power back.
At first, Sasuke would be filled with rage. His Sharingan was something he had cultivated and was a symbol of his Uchiha heritage. Naruto stealing it would be like stealing his very soul. He would immediately try to hunt Naruto down. But as he chases Naruto, he might start to think about the reasons why Naruto did it. He could consider if there was some greater threat that Naruto was trying to combat, but still, he would not be able to forgive Naruto easily and would demand his eyes back.
One possible fanfiction could be 'The End of the Uchiha' where Sasuke sacrifices himself to save the village from a powerful dark jutsu. In this story, his death is a heroic and tragic end to his journey.
Sasuke might be in shock at first. He can't believe that Naruto is dying in his arms. He may feel a great sense of guilt, thinking that he could have done more to prevent it.
The story could progress in many ways. For one, Sasuke might be filled with guilt for receiving Naruto's eyes. This guilt could drive him to become a better person and fight for the things Naruto believes in. He may also use the new vision to uncover secrets or powers that were previously hidden. On the other hand, Naruto, without his eyes, could develop other enhanced senses or abilities, and together they could form a new kind of partnership. They could also face challenges from those who oppose the idea of Naruto sacrificing so much for Sasuke.
One possible reason could be that his eyes hold a special power that he doesn't want others to discover easily. For example, it might be a new type of Sharingan or a unique ability related to his Uzumaki heritage that he needs to keep secret to protect himself or those around him.
Naruto would be devastated. Sasuke is his closest friend and rival. He would likely go into a state of deep mourning at first. He might isolate himself for a while, just trying to come to terms with the loss.
One possible reason could be that he overestimated his abilities during a battle in the Wave arc. Maybe he took on an enemy that was much stronger than he thought and got fatally injured. Another reason could be that he sacrificed himself to save Naruto or someone else. It's also possible that there was some sort of betrayal within the group they were with, and Sasuke was caught off - guard and killed.
It could be an unknown, ancient evil that has been awakened. This villain might have powers far beyond what Naruto and Sasuke have faced before. Maybe it's an entity from a forgotten era that was sealed away long ago and now has broken free.
One possible scenario could be that they sacrifice themselves to save the village from an ultimate evil. They might combine their powers in a final, desperate move that exhausts them completely, leading to their deaths.