Largely unknown, but was willing to lead The Fallen for some period of time. Was also complicit in their crimes as well.
Having accepted the Red Queen's deal, Seir's neurology has been altered: the defense of Shin is foremost in his mind though not strictly obedience to his new monarch. However his Queen also cured his cancer, thus he may have more a than typical attachment to her.
Coronzon was jailed alongside the other Fallen leaders like Seir, in his case he was likely in the medical wing given his condition. Following the prison's destruction he departed to Earth Shin and received medical attention from The Red Queen.
↑“Everyone! You have a choice! We are going to Earth Shin! We are going to be an authority [...] There will be rules! This means submitting to my power! It will not be as Goddess’ was! You’ll follow a code of laws, you’ll maintain control and peace, and you’ll protect populations. You’ll be reasonably good! Or you can stay here. You’ll be freer, but you’ll also be a target for heroes!” [...] Some prisoners were fairly quick to leave to follow. Ones who knew her from the Birdcage? With them went followers, and once a critical amount had left, a majority followed. Only the heroes really stayed. - Excerpt from Gleaming 9.15
↑Amy ventured toward the villains, but she wasn’t sure she trusted herself. She didn’t trust them, and she’d placed biological imperatives in each of them, to quiet bloodlust and regulate emotion. The last week had been her final push to get that done. - Excerpt from Interlude 16.z II
↑“No extreme violence,” Amy said. “I gave you rules and biological imperatives. Follow them.”
“I am,” Seir said. “Shin set their own rules and imperatives, remember? You struck your deal with them, you gave us the rules they dictated. We can do whatever’s necessary to protect Shin. Including if we think a mass murdering little shit like him might pull something.”
“Dangerous, horrible little shit like him,” Ahrima said, her voice young.
Rain was stone still.
Amy was silent.
She didn’t have nearly as much control over her rotten eggs as she liked to pretend. - Excerpt from Sundown 17.8
↑Coronzon would be the senior member of the Fallen group when Mathers was captured. From what little I’d seen of him earlier, he’d been someone who was very like Mama Mathers in how he’d moved. She had been so thin and malnourished that she’d looked like she would creak when she moved, a wisp of a person who came across as old when she definitely wasn’t. Coronzon, as Rain had suggested, had had cancer, and had been ill enough he’d hobbled.
He didn’t hobble now. He moved like a young man. He wore a shawl of animal hides that had been bleached as white as hides could get, over white robes that had stains around the ankles. A hood covered his head, and a thin black mass bubbled and twisted in the expanse beneath that hood, casting his face in permanent but inconsistent shadow that made it look like his skin was boiling. It didn’t follow his face perfectly, and at times he moved his head or glanced over at the heroes, I saw someone with sharp features and a sharper glare. - Excerpt from Sundown 17.8
↑I saw three or four of the Fallen capes in the mix. Complicated. They were some of the ones who had been in the background during the raid on the camp, periodically using their powers. If they were here, they were diehard enough to be deemed irredeemable. Most of the others had either expressed genuine remorse or they’d feigned it well enough to convince a judge.
If they were here, they were the kind of Fallen who were unquestionable problems. - Excerpt from Beacon 8.3