Seth (Star vs The Force of Evil)

Seth also known as Seth of Septasis, is a hidden and potential main antagonist of Star vs the Force of Evil. So far, he is only mentioned in Star vs the Force of Evil: Magic Book of Spell but his action paved way to most of the event in the series.

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

According to Glogor and Eclipsa, he is often described as an extremist and like to hold grudge for centuries. 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.

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

Trivia

 * It is highly possible that Seth is the power behind Toffee's action in killing Comet Butterfly.