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+ % Heal Mastery Increases the effectiveness of any healing charms you use by the % listed. At the time of this writing, the stats visible on the heal charm will not change, however.
-taken from the stat section in the LS portal.
Healing mastery used to be a valuable stat when the game was very different and your catacombs party had dedicated healers. At this point it expected that casters deal damage and heal. So Intellect is a preferable stat since it boosts your attack charm, heal charm & provides a defensive boost vs magical attacks.