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. Though with Toffee beaten off for good, in the canon series this never got to play out, thus Seth would remain a mystery....
Role in the series
Seth is revealed to have known about Mewni history longer than any other monster in Mewni history and has been aiming to make himself an avatar for The Xexaxez, and almost achieved it during the reign of Comet Butterfly, but he turned into a Xoldier cause he used a broken tablet that depicted the Xexaxez and the job was instead given to Toffee. Seth has been the opposite of his worst enemy Prime Major Admiral Asper Swipe, wishing to wipe out Mewmans with a great prejudice, and was even in strategy and power to him. But his discrediting has made him more determined than ever to wipe out all Mewmans, especially since Toffee ultimately failed when Star Butterfly made a new version of Eclipsa's nameless spell that can kill Outer God creations by spending extended time in the Neverzone. Seth would eventually have his Xoldier form taken from him by Asper in an attempt to become an Avatar to destroy monsters, but would fail and become a Xoldier instead, allowing Seth to cause some trouble of his own, declaring war on Mewmanity even when Asper restored the dreaded Solarian Program that was the darkest era of Mewnian history for both Mewmans and monsters.