Was Pelagius Septim the Seventh the real last Septim?

[Edgeheaded]

We all learned that Uriel Septim VII was the last Emperor of Tamriel directly related to Talos/Tiber Septim, but is he really? If you are me I say the real last Septim Emperor was Cassyndar son of Katariah who was a Dunmer, When Katariah's Septim husband was killed she became the empress and she was succeeded by her ill son Cassyndar, with his death his half brother Uriel Septim IV (Originally Uriel Lariat) lead Tamriel.

See? Uriel Septim IV wasn't a real Septim and all the emperors following him were his descendants... That means Uriel Septim may have been the last Lariat for all I know but certainly not the last Septim since he wasn't even one.