The Terror is a supervillain originally thought to be dead who operates in The City.
As recounted by a City police officer, he once sent his soldiers to slaughter the feline family of Mads "Murph" Murphy (aka Cat-Man-Dude) after his identity was exposed to the public by the authorities following his arrest on charges of mauling people outside a YWCA in Hoboken, New Jersey. To prevent more incidents like this, the government created the 28th Amendment, or "Murphy's Law" to keep other superheroes' true identities from being revealed.
Fifteen to twenty years before the present day The Terror used a weaponized and lethal version of syphilis to destroy the eyes of the Flag Five. He then had his henchmen kill most of them in front of a young Arthur Everest – who had just lost his father when the Flag Five's aircraft crashed on top of him after being hit by the missile containing the fatal virus. To top it all off, The Terror then mocked Arthur and stole his ice cream. This scene was photographed for an edition of the Times magazine which also detailed the Flag Five's deaths. Some time before this, he gave his protege Janet (the future Miss Lint) her power-controlling bracelets and also told her to own up to the nickname people had always given her.
A year later The Terror was seemingly killed by his immortal enemy Superian, whom he had engaged in battle with since the latter's arrival on Earth. The site of their last battle became a crater and The Terror was only identified when they found his teeth embedded in the crater walls.
Despite The Terror's seeming death many, including a now-adult Arthur, The Tick and former AEGIS agent and vigilante Overkill (actually surviving Flag Five member Straight Shooter) believed he was still alive, having gone underground to control all the crime in The City by using the likes of lesser criminals such Ramses IV and the Pyramid Gang to do his bidding.
The Terror is a cold, ruthless and maniacal supervillain who shows an absolute disregard for the lives of anyone but those who join him in his crusade, and even then only his most loyal and skilled followers, as seen during his massacre of the Flag Five, a confrontation with Arthur and a conversation he once had with fellow villain Miss Lint before he presented to her the bracelets she would use to control her powers. His elderly appearance belies his true cunning genius, as evidenced in the modus operandi that he employed over the years.
He has also shown himself to not be a villain without a sense of humor, even if it seems on the twisted and morbid side.
He follows the Music of the Spheres philosophy, basing his plans and actions on the steps of a song. However, this makes his plans seem stupid, absurd, or nonsensical. At the end of the season, the Terror is shown to hold a nihilistic worldview, seeing everyone's lives as a joke and that everyone lives in a meaningless existence.