Arcturus

 Arcturus is a Serpentine General who had a battle with Sensei Wu and Garmadon a very, very long time ago. It is unknown what tribe he is from and what he looks like. He may or may not have had a Serpentine Staff.

There is also a rocket named Arcturus that was used by the Digital Overlord, another serpentine, and the nindroids to retrieve what remains of the Golden Weapons. It is possibly named after the serpentine himself.

Project: Arcturus
Project Arcturus may or may not have some connection with the Serpentine General, but it is probably referring to the rocket with the name Arcturus.

Trivia

 * Being a Serpentine, Arcturus may hint that there (weren't) aren't just five tribes and there may be more.