Muria used source power to cheat and enhance his own strength, but he did not use source power to directly increase it. The reason his power could explosively grow was due to the Heart of Balrog he possessed.
However, the Heart of the Flame Demon was considered a foreign object in this world, deemed a "forbidden item." Although its power was partially compatible with Muria's Yama, normally, integration would still not be possible.
At this time, Muria used source power to overcome these barriers. The slight amount of source power he expended allowed the Heart of the Flame Demon not to be rejected by the world, and also eliminated the rejection between the Heart of the Flame Demon and his body.
This consumption was much less than if Muria had directly used source power to increase his strength, making it the wisest choice. Directly using source power to increase strength would be too naïve!
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