From Serenes forest, I can't link because it has scans from this week's famitsu, but it basically explains the Bloom Palace video:
At the Bloom Palace, players can chat with the heroes’ Mirages or speak to Tiki to perform “Carnage Unity” or “Radiant Unity”.
Both forms of Unity require energy known as “Performa“, which can be obtained from defeating enemies; naturally, different enemies drop different types of Performa.
Players are also encouraged to exploit enemy weaknesses to trigger session attacks, as doing so increases the amount of Performa gained at the end of battle.
As we learned before, Carnage Unity uses Performa to create weapons for Mirages, each with their own attack rating, elemental resistances and obtainable skills.
Meanwhile, Radiant Unity uses Performa to grant “Radiant Skills”, which include those that improve stats or other skills or add new session attacks, etc.
Lastly, while at the Bloom Palace, players can use Master Seals obtained from battles to class-change their heroes’ Mirages to one of two upgraded classes.
Below are some example class paths.
Chrom: Lord to Great Lord (healing and support skills) or Conqueror (high attack and defence).
Caeda: Pegasus Knight to Falcon Knight (improved healing) or Dracoknight (weakens enemies)
Cain: Cavalier to Paladin (well-balanced attack and defence) or Dark Knight (fire-based and enemy-weakening skills)
Draug: Knight to General (superior defence plus healing) or Berserker (exchanges defence for offense)