The reason Damien had to clear his subspace was equally as simple as everything else. The coming processes would be relatively dangerous.
Not for any of the people involved, but for his items. If he made a mistake, there was a high chance that they'd be thrust into the void and consumed by its nothingness.
Soon enough, various beast carcasses, weapons, pills, and random items that would probably have use at some point spilled out onto the cave floor.
Even Damien's sword, Devourer, which he hadn't been using much was within the pile.
Once he was done, he quietly sat down in a meditative posture and focused on the space around him.
There, tethered to him was a small pocket that would go unnoticed by the untrained eye. It wasn't big at all, perhaps the size of a quarter at most, but the space inside was expansive. This was the nature of a subspace.
'Good. That was probably the only easy part of this whole ordeal.'