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I think it would be better to change the artifacts because most players will choose the Mirror, the Wizard's Sign, and Artifact Absorption instead of the Golem March or Machine Loyalty.
It would be better to give the Cursed Sign back to the Demons and make it so that it's not applied directly, but rather, if an enemy kills a stack of Demons, they receive the Sign (like a black mark). This would make it easier for Demons at the beginning (when using a single-unit tactic) and a little more difficult at the end, in the final battle (but if the bonus damage is (hero attack) * 100% (50 + 4 for each hero level), it will be balanced).