Xue Wei could feel how his spiritual energy was slowly diminishing, but unlike usual, he knew that he could no longer replenish it as before.
When cultivating any kind of energy, the expert would be trying to progress through various layers of strength, and for each layer, the cultivation base would change.
The cultivation base was the core of their strength. Even if Xue Wei used all of his Qi or his spiritual energy in normal situations, he would merely need to wait or cultivate for the energy to refill his body once more.
However, the strength one could wield would be based on the layer one's strength had reached, so this layer of cultivation was known as the core.
For spiritual energy cultivators, they indeed had a core within their bodies. It was a bit similar to that of spirits, but humans and beasts would never be able to control a spiritual energy as pure as these spirits, and thus their cores would be weaker and smaller.