The blank air rune is a quest item used in The Slug Menace. It is made by using a chisel on a normal rune essence or pure essence after adding the missing Page 1, Page 2 and Page 3 back into Brother Maledict's holy book. During this process the player must also make a blank earth rune, a blank fire rune, a blank mind rune and a blank water rune.
Make more than one blank because you can fail while chiseling the shape of the rune, and/or binding it with the element of the rune.
When used on an Air altar in the usual way it turns into a special air rune which is needed later in the quest.