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With the help of Emeraldine, Root, and Ash, and well, Jonathan for more purifying stuff, alongside my near endlessly regenerating Mana, we got to work.
First of all, we helped Ash manipulate his Mana, and to "feel" the soul within him. He slowly began to imbue this soul's powers into the contaminated soil, purifying it easily.
With Emeraldine and Jonathan's aid, the soil was purified rapidly as long as I gave them Mana, and after just a single hour, the entire contaminated area was completely purified.
The dead trees remained dead though, they were completely soulless, the souls were probably sucked away by the evil god fragment, so there wasn't anything we could do about those.