He might face internal struggles. Since it's the power of his enemies, it could have a corrupting influence. He has to constantly fight against the darker aspects of that power, like the urge to use it for revenge or in a more aggressive and unethical way.
In other cases, it might be a result of a curse or a spell gone wrong. His enemies could be trying to harm him with a dark spell, but instead, it backfires and he ends up absorbing their power. For example, if they were trying to drain his own power but the spell reversed on them.
Since I haven't read the 'orphen gains dragon power light novel', it's difficult to determine the main theme precisely. But from the title, it could be centered around Orphen's journey with his newly - gained dragon power. This might involve themes like adventure, as he likely has to face new challenges with this power, and perhaps a bit of mystery regarding the origin and nature of the dragon power.
In some fanfictions, Percy might gain power in Tartarus through facing and overcoming numerous deadly challenges. For example, by defeating powerful monsters that inhabit Tartarus, he could absorb some of their essence or gain knowledge from them which then transforms into power. Another way could be through making pacts with certain entities in Tartarus in exchange for power, although this would likely come with a price.
He might gain enhanced perception. This could allow him to see chakra flow more clearly, similar to how the Sharingan can. So in battles, he can anticipate his opponents' moves better.
Percy could gain power by defeating the most powerful beings in Tartarus. By taking down the toughest Titans or ancient monsters that lurk there, he could absorb their power or gain respect from the other inhabitants, which would help him rule.
In this fanfiction, Percy could gain his extraordinary power by facing and overcoming numerous challenges. Each battle he fights, every enemy he defeats, could leave him with a new ability or strength. It's possible that he has a unique connection to an ancient and powerful force, like an old primordial energy that he manages to tap into, which gradually makes him more powerful than Chaos.
One possible source is the dark magic or ancient spells that exist in Tartarus. Percy might stumble upon a spellbook or be taught by a long - forgotten spirit in Tartarus. Another source could be the unique minerals or substances found in Tartarus. If he consumes or uses them in some way, they could enhance his abilities. For example, there could be a crystal that amplifies his water - based powers.
It could be a corrupted demigod who wants the Power of Dominion for himself. This demigod might have been overlooked for too long and feels that with this power, he can finally get the recognition he deserves, but in a very wrong way.