In addition, Khepri declined to capture him while collecting parahumans during the battle against Zion, believing him to be "more trouble than he was worth".
His power seems to be related to some form of matter creation, which uses brains or perception as an attack-vector, since Wardens sought out All-or-Nothing brutes with resistant/non-typical brains and absolute matter annihilators to deal with him.
↑“I don’t suppose any of you feel brave enough to go up against Sleeper? Absolute invulnerability better than Alexandria’s, special brains, absolute annihilation powers?” - Excerpt from From Within 16.2
“Restless but still around. Earth Zayin in the palm of his hand. He started to emerge, Legend faced him down. [...] Leveled about thirty square kilometers of city.”
I nodded. “He’s dormant now?”
“For now. If he had motivations for making one attempt, he’s going to budge again soon. But I don’t have the instinct he’s our threat. He’s background. Someone else’s problem.”- Excerpt from Sundown 17.9
↑"Wildbow: Sleeper was written in as a response to a fan saying that I shouldn’t go into detail and explain every character. He featured in a snippet I wrote back in the day (I spent ten years writing bits and pieces of superhero stuff before settling on the story I wanted to write) and seemed like a good one to drop in. He does have a power, but I think it’s best left unsaid, perhaps until the sequel." - Interview with Wildbow by t4nky