This requires a contest of detection/analysis ability vs. the caster's skill with this spell to pierce through the false magical aura. If cast on a magic item, this can either hide the existing aura completely, or obscure it by making it appear to be something it isn't.

This can also create other types of illusion effects that would only be visible to those with magical senses.

Illusion [62] 
    Affects Abilities to Sense and Analyze Magic [+0%] 
        This is instead of vision and hearing 
    Independence [+40%] 
    Persistent [+40%] 
    Extended Duration (Permanent) [+150%] 
    Based on Activation Roll [+5%] 
    Takes Extra Time 6 [-60%] 
    Magical [-10%] 
    Spell Components [-20%] 
        Must say "magic" in Latin 
        Must gesture with hands 
Levels 2-7 [64/67/69/72/74/77] 
    Reduce Takes Extra Time by 1 level [+10%]