1. Samehada·Human Body Fusion: This is a ninjutsu where Kisame and Samehada combine to gain powerful power by absorbing chakra. However, if there is not enough chakra, there is a risk of going berserk. Kisame is nicknamed the "tailless beast" because of this combination. 

2. Inuzuka-ryu Human-Animal Hybrid Transformation: The Inuzuka clan possesses a secret technique that allows them to transform into giant wolf forms when combined with their ninja dogs.

Although they have powerful activated fangs and super fangs, these abilities have limited effects against strong enemies.

3. Powerful Susanoo: By combining Susanoo and Nine-Tails, the "powerful Susanoo" is created, which is stronger than the individual-tailed beast and Susanoo. Naruto and Sasuke successfully fused in the Ninja World War, creating an awe-inspiring appearance stronger than Madara's.

It is considered the strongest fused ninjutsu. 

4. Kiyaro no Jutsu: This S-level forbidden jutsu, created by Beiruhu in the movie version, allows the absorption and fusion of other ninjas and even blood successors. The combined psychic beast is powerful, but fusing with a psychic beast may appear strange. 

5. Secret Technique: Attack of Two Demons: Ukon's blood inheritance secret technique allows him to merge with enemies and erode their cells. Kabuto Yakushi controversially uses this secret technique to merge with other people, which is considered disgusting.