The Mane 6 in their Rainbow Power forms S4E26
The Mane 6 rainbowfied.
Twilight and friends return to the chest, where Twilight's medallion turns into the last key. Before Tirek discovers and stops them, the six friends turn the keys at the same time, and the top of the chest opens, releasing a rainbow-colored blast of power that showers them in light. The ponies are transformed and given rainbow-marked coats, manes and tails. They confront Tirek, who is unable to overpower their magic.
A blast of light strips Tirek of his power, and he is banished to Tartarus once more. Twilight and her friends fly over all of Equestria and return all of the ponies' stolen magic, and the other princesses are freed from their own imprisonment. Finally, the Tree of Harmony launches the chest and its keys toward Ponyville, where it sinks into the ground. Mere seconds later, a great castle rises out of the ground from where the chest landed.
Friendship Rainbow Kingdom glowing radiantly S4E26
Twilight's kingdom appears.
Twilight and her friends behold the castle's majesty as their rainbow powers wear off. Princess Celestia then appears, telling Twilight that the castle is hers. As they walk through the castle corridors, Twilight finally understands her role as a princess: to spread the magic of friendship across Equestria. In recognition of her role, Princess Celestia designates her the "princess of friendship". In the castle throne room sit seven thrones, one for Twilight and each of her friends (except for Discord, who Fluttershy tells has yet to deserve one).