IKonIK
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There are a lot of tactics on this topic, and in 90% of cases, it doesn't make sense for the first hero to take Diplomacy. For example, Kira from the Tower – what good is Diplomacy to her? Some Crag Hack will beat her, even though he has three times fewer stats. However, I try to develop it for secondary heroes as much as possible. These artifacts, as DmitriyAS said, are needed in very rare cases, BUT they are needed!