The problem of the death spirit has been solved, but the issue of the [True Dragon Water Palace] is far from resolved. The Ancient Well is a manifestation of the problems plaguing this palace. Kill one death spirit, and soon enough, another will be formed in the well.
The True Spirits dwelling in the treasure house are not incapable of resolving these issues. Rather, they're a part of the problem itself and require external assistance for its resolution.
Owen Bertram infused the well with Spirit Fire, gradually "cleansing" its entirety. Thanks to his use of the [Abyss], the resentment and particularly the dragon-shaped black vapors condensing the death spirit have already been devoured, removing most of the power remaining in the well. So it's not too arduous a task for Owen.
However, progress is still slow. It took Owen four shichen to purify all of the well's water. Now, this place no longer serves as a gathering spot for resentment.