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Harm is essentially damage without elemental alignment. Perhaps it is a left over from the earliest times of creation when not everything was infused with elemental power, perhaps its from an alien world that obeys different rules, or perhaps its meddling with truly advanced magic/science/magiscience.
Harm as an element in battle typically ignores the resistances of the eight standard elements and strikes with an element that has few methods hindering its damage aside from blocking. Status effects dealing damage in a non-elemental form like Poison deal Harm damage so that the full amount of damage would always be done. With a few exceptions, a player's and monster's resistance to Harm is always 100 percent.