The Mundane Standing Stone is a block added by Roots. It is used to perform advanced Rituals. Rituals requiring them use eight Mundane Standing Stones placed around the Casting Altar.
Advanced Rituals requiring the addition of Mundane Standing Stones are arranged like so.
— Runic Tablet
In order to gain access to more powerful rituals, you need a better way to channel life energy from the world around you. By carving runes into this stone structure, you give it the much-needed ability to draw in power from the world around it to fuel your magical desires.