Seth of Septarsis

Seth of Septaris is a Septarian from Mewni and a potential major antagonist of the cartoon series Star vs. the Forces of Evil. So far, he is only mentioned in the book Star vs. the Forces of Evil: Magic Book of Spells, but his action paved the way for most of the events in the series. He is the same race as Toffee, and he has the ability to regenerate from any form of intense damage.

History
Seth is the oldest Septarian with a long hatred of Mewmans and wished to eradicate them from Mewni.

According to Globgor and Queen Eclipsa, he is often described as an extremist and likes to hold a grudge for centuries much like the rest of the Septarians. Moreover, Comet Butterfly described him as an old and decay monster with a decaying and rotten hatred and ideas but dangerously attractive to the young generations of the Septarians. Most of the peaceful monsters reviled and feared him as even Globgor, the Prince of Darkness, calling him troublesome and despicable. Furthermore, he is also the reason why Eclipsa developed the darkest spell to combat his race.

During the conflict between Mewman and Monsters and the reign of Star's grandmother, Comet Butterfly, he led the monster faction that opposed the peace treaty made by Comet Butterfly and Archduke Batwin. When Comet tried to invite him to the banquet to settle the conflict peacefully, he declined and wanted her dead.

It is unknown what happened to him after Moon defeated Toffee but with Toffee stated that he had a more hidden plan. Therefore, it is assumed that the plan will involve Seth.