The mirror switches visages with the target, and the one in the mirror ain’t pretty.
When a target is within 30′ of the mirror, they must make a DC 14 Wisdom saving throw or be compelled to walk towards the mirror and look into it. When the target is within 5′ of the mirror, they get a second DC 16 Wisdom saving throw. On a failure, they switch faces with the ugly person in the mirror. This causes the target’s Charisma bonus to drop to -3 (although their score remains the same for the purposes of ability damage).
While walking towards the mirror, if another character tries to intervene, the target can make a second DC 14 Wisdom save to stop.
Once the faces have been swapped, the target can only be put back be a remove curse. When that spell is cast, the face inhabits the nearest mirror, and that mirror is now identical to the one that cursed them.
A detect magic detects an aura of transmutation on the mirror. A DC 20 Intelligence (Arcana) check reveals an arcane presence inhabiting the mirror.
The mirror can be neutralized with a dispel magic, however, as above, the face moves to the next nearest mirror. In order to keep the face from moving, the only way is to banish the mirror to another plane, as the face cannot jump across planes.