I am currently questing in Mirage and I have a quick question about the Arcanum Apartment.
I used a housing elixir so that now I have 4 slots. But I only have 3 houses (3/4). I am wondering, when I get the Apartment, will it use up my empty 4th slot or will I get a 5th housing slot (4/5)?
I would be very unhappy to have paid for a housing slot which automatically gets filled by the game.
When I received the Arcanum Apartment, it automatically gave me an elixir and then filled the new slot (4/5 now).
And in reply to the question, you get this Apartment after completing the quest called "Trial of the Understudy" which comes near the end of Mirage (but before you battle the last bosses), around level 118 or 119.