Flensse was the best 'interrogator' of the Universal Church of Truth. Basically, Flensse tortured someone until they converted to their faith. He used a weapon known as a mnemonic flail that stripped memories away from his victims with every stroke.
After Magus had imprisoned some members of the Guardians of the Galaxy, he ordered Flensse to convert them. As Flensse lashed Phyla-Vell, she was enraged so much that she broke free of her shackles. She finished his life with her Quantum sword.