The Slug is a monstrous cape who can brainwash people. He erased the memories of Case 53s, and also was, likely, able to transfer knowledge of languages to his subjects. It made him a key part of Cauldron's nemesis program.
The Slug was crucified by a member of the army of monstrous parahumans using chains when Cauldron fell. Taylor and Imp both assumed he was on the verge of death, although neither actually saw him die.
Estimation: Poses a threat only just above human norms, an unexceptional individual may be able to cope with the ability or walk away unharmed from an altercation where the abilities in question were leveraged. Ability should be assumed to be a nonthreat and need not be identified if other matters prove more pressing. [...] Master Has the ability to control others. Can include degrees of control (swaying attitudes or emotions) and a wealth of controlled entities (individuals, animals, objects, created beings). - PRT Master Reference - Classifications
↑PitaEnigma: So basically Contessa refused to talk to anyone other than Doctor Mother PitaEnigma: and people outside of Cauldron? Cyrix: eeeeh Cyrix: I am unsure how and why you jump to that conclusion Mishie: Uh PitaEnigm>: Because it's a funny mental image, mostly Mishie: Wat Cyrix: thats not a very good reason Wildbow: Not refused. Just wasn't necessary. The Triumvirate would've felt like their hands were being forced if Contessa got involved. Wildbow: Doctor Mother speaking is a little less threatening & a little more organic. abyssonym: that's right, they agreed to let her do most of the talking Wildbow: They're ~surrounded~ by mostly mute people acting as tools. Wildbow: The clairvoyant, doormaker, the custodian, countless deviants. Wildbow: The slug. - Wildbow on IRC, archived on Spacebattles
↑ 5.05.1The roaring reached a climax. They had a man with no arms or legs, not fat, but with a goiter-like mass around his neck… hairless. A case fifty-three.
“This one,” Imp said, repeating what the mob’s ringleader was saying. Shouting, judging by the way he was acting on camera. “This traitor, he is how they controlled us. How they planned to control you. He was going to brainwash these ones into a private army… he’s pointing at the weird looking ones they brought from downstairs. This traitor was going to send the rest of you out without any memories, without identities, as Cauldron’s trash.” - Excerpt from Venom 29.5
↑The plans had failed. Only Khonsu and the Indian capes were still active. Capes brainwashed with a deathwish, working in coordination with an Endbringer who could move them to any location instantly, and who could theoretically block some of Scion’s attacks. - Exerpt from Interlude 29
↑“It means a great deal,” Alexandria said. “We lost a great deal of power, leverage, trust. The heroic organizations are going to be sundered by this knowledge. Try as we might, we can’t erase their memories.”
“No,” Doctor Mother said.
“Unless you wanted to use the slug?” Alexandria mused.
Doctor Mother shook her head.
“The slug,” Legend spoke. “I was wondering how the case fifty-threes came to lose their memories. Not something of Manton’s, because he wasn’t involved in making them. It’s yours.”
↑She knew English even though she didn’t know why, like she knew Russian. This was her first time using it. - Excerpt from Black 13.z
↑“There are backup plans if the whole parahumans-as-leaders thing didn’t work out. Brainwashing leaders like they brainwashed the case fifty-threes. So the leaders were absolute and could be trusted. Um. Distribution and organization for getting things going again, depending on how many threats remain after we make it through this. They didn’t know what the end would be like, what we’d be up against, so they could only ballpark here. The reason for these offices? Cauldron’s going to staff this place. It’s going to be a hub, police, a whole lot more, up until humanity’s got the ball rolling again.” - Excerpt from Venom 29.4