They would frenziedly attack the living, biting open their blood vessels, sucking their blood and flesh, likely subconsciously 'replenishing vitality'. And each of these 'walking corpses' became nests that the Dao Spirits could seize and dwell in. As the number of infected surged exponentially, hordes of zombies began to erupt by the hundreds and thousands, making even superhumans like Zhang Boren and Li Pan struggle to cope.
Those Dao Spirits took the opportunity to drill into the corpses, and at a speed visible to the naked eye, they began breaking through their cultivation, strengthening their Dao bodies.
Just like Li Pan practicing the Divine Sect techniques, in a few breaths, the body would evolve to a fourth-level strength, achieve 'Foundation Establishment', and continue to rapidly grow in cultivation, breaking towards the fifth level. No, not 'breaking through' but 'recovering' their power!
