Anima Gate of Memories: The Nameless Chronicles

MISSABLE

To defeat the Secret Boss you have to open the Inner Halls of Arcane (Secret Room trophy) and open the central passage after collecting all five Nameless Memories. Creature on the throne inside will give you quest to find five of its' friends and provide location to them, which I will post below for ease:

Demon Bird Mask: Laberyntos
Shaman Mask: Exterior Wall
Knight Mask: Mansion Basement
Strange Mask: Outskirts of the Village
Cat Mask: Etheldrea's Mansion

After you talk with all five, creature will ask you to kill it so attack it until it is out of life points, boss fight will start after that. I should point that boss fight is quite tough, thus I recommend doing it as late as possible so you have the best gear and maximum level obtainable. Also better do it on your first playthrough, as on New Game Plus I'd give this fight difficulty is 10/10.