The victim must make a Constitution Save.  If they fail the save, they are unable to use any non-legendary magical abilities.  They may use magic items, including potions, wands or scrolls, but can not cast any spell or use any other magical racial ability.   If it would not work while they are in an anti-magic field, it won't work for the duration of the spell.

At the end of each round, they get a new Constition Save to end the effect.

 Higher level casting:    If upcast to 6th level all saves after the first one are made at disadvantage. At 7th level, in addition to the disadvantage on the 2nd greater saves,  once they fail three saves they get no more saves.   At 8th level or higher, the spell no longer requires concentration. At 9th level they only get a single save.