Gilvas Barelo, "[...]We don't know what happened, but this is what we believe. His experiments with Kagrenac's Tools had joined him to the heart's divine nature in some way, so that he learned to draw power directly from the heart.
We conjecture that Dagoth Ur, driven by anger and greed, used the heart without caution and restraint, and, as a result, he has become terribly powerful, and terribly mad. But the Tribunal showed great care and restraint in their use of the tools, and so they were not driven mad [...]. Nonetheless, the Tribunal, too, appear to heave been corrupted by the heart's power, though more subtly.
[...] The Tribunal [has lost] its battle to control the power of the heart. They are sustained by the same tainted power that drives Dagoth Ur mad. [...]"