Inner Fire All Creation Refinement
In the Daybreak Tower, Zatiel has been sending his golden flames to the spine for days. His Eye of Life and Creation was monitoring the process and arousing the special properties in the flames.
The process had to be done carefully since any damage done to the runes in the Pit Fiend's spine will be irreversible which will weaken it tremendously, and making it lost most of its abilities.
Even with the constant replenishment of his energy pool by the magic tower, Zatiel was growing more and more tired, but he did not stop and continues advancing.
It was at the end of the seventh day when the blue liquid had completely evaporated that the purge of Hell Aura from the spine was over.
The mind corroding power was still present but the foul energy in it has vanished and now you could see thin golden lines running through it.