Once the 2 years were over, Alex ran out of the resources he had. As such, he moved swiftly to using the incubating Sun Wraiths. These were beast cores that were contaminated by Sunhearts, which then turned the spirit inside the beast core into a wraith.
Any core from an Immortal beast formed an intelligent Sun Wraith. As such, Alex chose to use the ones he had made from the mindless beasts he killed back in Hell. A single such beast core gave him enough Energy to last several weeks at a time.
At the end of those weeks, Alex resorted to regular cultivation, which helped him pass an entire month with a single core. The speed didn't drop whatsoever either, but with every month that passed for him, another Sunheart was being wasted.
That was another Sunheart that could have become a Blood Beast, one he could have added to his army. Instead, he was wasting it all here.
However, between Blood Essence and Blood Beasts, the choice was quite obvious for Alex.
