The masks the DUF is using to brainwash monsters are called Turn-Masks, since they, simply put, turn monsters against each others. The DUF uses them to let the “turned” monsters fight for their cause.
Officially the Turn-Mask technology was salvaged from King’s Cradle when it fell after the DUF’s siege. This statement however is questionable, due to unofficial comments on the war campaign beforehand. According to those, turned monsters fought under the DUF comand even back then, many years before King’s Cradle was raided. It is yet to be determined if these claims are truthful or made up, but since the winner writes history, I tend to believe in the critics.
So far I couldn’t find out how exactly the masks work, but magiqe doesn’t seem to be involved. They seem to be highly sophisticated electrificy devices, crafted individually to match the assigned monster’s physical and mental abilities.
Very little is known about the mental state of “masked monsters”, but they seem to be conscious at all times. One miscalculation or error can apparently lead to a grave malfunction and therefore disable it completely, setting the individual free. No records exist about freed monsters through malfunction or other means, though, which leads to the conclusion that the DUF is holding them under high security measures.
The masks apparently make monsters accept any command given by their handler or handlers. It’s indistinct if some sort of telepathic link is established, but thoroughly thinkable since nothing is known about the process.