The blank water 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 Maledict's holy book. During this process the player must also make a blank air rune, a blank earth rune, a blank fire rune, and a blank mind 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 a Water altar in the usual way it turns into a special water rune, which is needed later in the quest.