Marx Soul is the zombified second form of Marx, appearing as the final boss of Kirby Super Star Ultra's The True Arena.
After his initial defeat by the hands of Kirby, Marx fell to his supposed death when he was sent plummeting towards Galactic Nova. Kirby learnt that this was not the case when he encountered Marx again in The True Arena, now sporting a more maniacal appearance; it was stated that Marx actually absorbed the pieces of Galactic Nova and became much stronger than before.
In M.U.G.E.N, Marx Soul has been made by Chop23. It plays exactly like it does in Super Star Ultra, but it cannot actually deal damage. It is presumed that the creator wanted someone else to handle attack damage and balancing, as evident by "the one who takes this over" being the internal author name of the character.
Marx Soul is a very early prototype that, while not being able to deal damage, is fully functional in all other aspects, being very source-accurate in his attack patterns. Intended as a boss character, Marx Soul can only be controlled the A.I.
This character is controlled solely by the A.I., so no activation methods are present.