Lightning Release + Water Release.
The fusion of these two chakra natures would create a new ability: the Storm Release Bloodline Limit.
Legend had it that the Land of Lightning possessed this bloodline limit, allowing straightforward Lightning Release ninjutsu to move like water and turn at unimaginable angles.
Hatake Sakumo was a genius.
However, the White-Haired Youth was ultimately still too young, and his chakra reserves were limited by his age and body. He usually preferred practicing swordsmanship, so he was somewhat at a loss when it came to training in chakra nature transformation.
Fortunately.
Shirou Emiya was a good teacher.
As a ninja proficient in chakra nature transformations of every attribute—someone who had even glimpsed the essence of chakra itself—guiding someone to master Lightning Release and Water Release chakra nature transformations was relatively easy for him.
Compared to other ninja instructors, Shirou Emiya could directly guide Hatake Sakumo step by step through chakra transmission, helping him grow accustomed to the two chakra nature transformations.
This method of teaching carried enormous risks.
Normally, chakra would often go out of control and injure the body, but solving such minor problems was child's play for Shirou Emiya.
Hatake Sakumo's training progressed rapidly. Unfortunately, bloodline limits were not so easily fused. Still, mastering Lightning Release and Water Release chakra nature transformations in such a short period was already proof enough of his genius. It also often earned him unhappy looks whenever Little Tsunade returned from school.
"I'm unhappy."
Little Tsunade watched Hatake Sakumo train with a pout. She was still struggling through her training course to master Yang Release Chakra. "Why can't I be as talented as Senior Hatake?!"
"The requirements for becoming a medical ninja in this world are extremely harsh, but that does not prevent those who cannot become medical ninjas