Mlp Equestria girls musical control

Twilight figures out the Secret of dimensions that involves the main six cutiemarks.After a few weeks living in the castle the map shows them a new location to investigate but only twilight cutiemark is in that area.It was in the crystal empire where the mirror to her human friends.While getting over there starlight glimmer brother uses his magic to remove twilight's cutiemark and runs off to sell it for money or bits.Twilight finds him and shows him what friendship really means he reaturns her cutiemark and gives her his prized possession a cutiemark necklace .She continues going to her destination when she reaches the mirror it begins to flow she goes through it and she goes to a completely different world then normally.
What is this world what will happen find out