Aatrox Mundo
So far, we've seen two major flaws with the Darkin's choices in vessels:
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In the case of Aatrox, the body is in constant pain and falling apart because it rejects his will
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In the case of Varus and Kayn/Rhaast, the mind of the host(s) is(are) able to fight back against the imposing will of the Darkin.
What would solve both of those? Give the Darkin Blade to Mundo. (Rewritten) Mundo's body naturally has amazing regenerative ability, which would combat the constant damage dealt by the imposing will of the Darkin. He also has low mental capacity, which would make it easier to take complete control without his mind fighting back.
Mundo as a Darkin would be an unstoppable monster.