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The Iron Whetblade is one of many Key Items found throughout Elden Ring.
The Iron Whetblade allows you to use the heavy, keen, or quality affinity when using certain Ashes of War.
This item is found in Stormveil Castle. Starting from the Rampart Tower site of grace, head out the door and take a running jump off the side onto a roof. Drop off the right side and use a Stonesword Key to open the fog door allowing access to the armory. On the body in front of the fireplace is the Iron Whetblade.