'So, accumulating energy…'
According to Harold, it could be done in multiple ways.
The easiest aside from stealing was still stealing.
People died often in this realm. Sometimes, people would even commit suicide and erase their memories so they could be reborn as entirely different people.
No matter how they died, they would always revive. The concept of death didn't exist in this place, but one's existence could absolutely be wiped by the world itself. It didn't happen often, but if a practitioner tried to touch things they weren't meant to touch, it was a method the realm would occasionally enact.
If a practitioner died without having their existence erased, their energy would disperse into the world. The fifth island especially was filled with this energy. It was a battleground for the Ancients who apparently reached the level of Nonexistence easily.