Feidilan was Feidilan, and the Zombie King was the Zombie King; they were two entirely different people, two separate concepts, and should never be conflated.
Long ago, before Feidilan became a demonic cultivator, his cultivation realm had stalled, and his lifespan was completely exhausted. In desperation, he explored ancient ruins, searching for the fortuitous encounters left behind by high-level predecessors.
Heaven rewards the diligent, and sure enough, he actually excavated a demonic cultivator's grotto-heaven.
Chapter 165