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Iron vrykul are a variant of vrykul.
They may be related to the Iron dwarves in some way. It is possible that the Iron vrykul are connected to the Curse of Flesh. This is made apparent by the fact(?) that if the vrykul were themselves once Iron vrykul (as most seed races were), the Iron vrykul found in the Halls of Lightning are themselves remakes of the original vrykul, only immune to the Curse of Flesh.
Notable Iron vrykul