Copycat

While holding the mirror out towards a creature that casts a spell, you can choose to try and copy the spell they cast, when you successfully do so, you cast the spell using a spell slot equal to the level of the spell being cast, (For example, if the spell being cast is of 3rd level, then the spell slot this spell uses is of third level instead of 1st). When you cast the copied new spell, you can choose who is targeted or attack who you desire with the spell, if follows all regular spellcasting features of that spell, however it uses the spell attack modifier and spell save DC of the original caster, (The GM or another player holds the stats required depending on circumstance).
If you attempt to copy a spell that is cast using a spell slot for which you do not have spell slots, the spell fails to activate and your spell slot is lost.