Jamieson, Fausset and Brown on Mark 5:4

4. Because that he had been often bound with fetters and chains, &c.--Luke says (Lu 8:29) that "oftentimes it [the unclean spirit] had caught him"; and after mentioning how they had vainly tried to bind him with chains and fetters, because, "he brake the bands," he adds, "and was driven of the devil [demon] into the wilderness." The dark tyrant-power by which he was held clothed him with superhuman strength and made him scorn restraint. Matthew (Mt 8:28) says he was "exceeding fierce, so that no man might pass by that way." He was the terror of the whole locality.