Eleven

Eleven (イレヴン, Irevun) is a group of 11 rogue espers that notoriously feeds children with psychic-induced drugs to create an army of espers. They are led by a powerful esper named Otento.

History
At some point in the past, this group was formed by Otento. This group then began performing sinister experiments and administering drugs to children to turn them into espers. It appears that Tatsumaki is affiliated with them, given that she was familiar with the leader. A police investigation was made to arrest this group, but the entire search party vanished.

Plot
They are mentioned by the Hero Association as plotting something sinister and assign Tatsumaki, someone who was also looking for them and who has a personal history with them, to eliminate them.