Board Thread:Theories and Speculation/@comment-6769189-20200705100921/@comment-32685487-20200705220549

https://naruto.fandom.com/wiki/Tenseigan

https://naruto.fandom.com/wiki/Tenseigan_(Energy_Vessel)

Tenseigan is a real dojutsu, and Hamura had it first as far as we know.

"A passage in the movie's novelisation can be read as the energy vessel having been created after Hamura's death, to preserve the power he wielded in life. Conversely, Retsu no Sho states that the energy vessel was created while Hamura was alive and at his direction.[1] As such, it is unclear whether or not Hamura possessed the dōjutsu version of the Tenseigan, and if he did what he used it for. "

Hamura most likely sealed his Tenseigan into the vessel first, making it the Tenseigan Vessel.. then other Byakugan were added giving the Vessel more power.