A multiplayer RPG real-time strategy you can play with friends is SpellForce 3. It's only available on PC, but without Mac or Linux support.
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{"name": "SpellForce 3", "genres": "real-time strategy, role-playing", "has_multiplayer": "yes", "platforms": "PC", "has_linux_release": "no", "has_mac_release": "no", "intention": "inform"}
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