The duplicate Eiro stepped up to the bedroom door, carefully placing his palm onto the door. There was a specific link placed here that allowed the original body's soul to be fully accessed from here, at least through the duplicates. It was basically just a relay point for Eiro's mana, like a lightning rod that the demon could specifically point his mana toward.
Like that, objects could be taken out from the treasury, the duplicates' bodies could be altered, new duplicates could be created, and like right now, the duplicates could receive new mana from the original. Since they were just duplicates, they weren't able to fully produce their own mana or access things like the spirit gateways through their own bodies, but the passive consciousness of the original could be triggered in ways that allowed the duplicates to access it this way at the very least.
