Atrenan's Marvelous Rune

You place a simple rune on a surface near you. This rune is five feet in diameter unless placed on a smaller object, in which case it will appear as large as the surface allows. When you place the rune, choose fire, lightning, cold, acid, poison, necrotic, or psychic. When a creature touches the rune, it takes 3d6 of the chosen damage type. You may take an additional minute to cast this spell; if you do so, the rune does not have to be attached to a surface and can remain at any point in space.