After dealing with Kaspar, Farcia immediately rejoined Nicholas and the others. Upon arrival, she found them securing four prisoners, all bound in black chains that emitted a strange energy. These chains were a special kind used specifically for restraining prisoners, commonly known as magic-suppressing chains, black chains, or dark shackles.
When fully bound by these black chains, the mages could not use their magic as they wished. In simpler terms, the black chains suppressed the prisoners' magical powers.