How do I unlock Iron Wizard mode?

//ads -- adsterra.com -- native banner
?>
According to Steam, the most elusive achievement is beating the game in Iron Wizard mode. This mode has a way lower skill-point cap than the normal play mode (because there's no wizard XP, just 5 skill points per beaten level).
I can't seem to figure out how to unlock this iron wizard mode though. What do I need to do to unlock this?
Pictures about "How do I unlock Iron Wizard mode?"



Gemcraft Chasing Shadows - Field U8 - Iron Wizard Mode - Open Tome Chamber
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: Ksenia Chernaya, Ksenia Chernaya, KoolShooters, Giallo