The Shatters Key open The Shatters dungeon.
The Shatters is an extremely dangerous dungeon, the concept for which was originally drafted by a player named LordShon in the old Wildshadow forums. The original design was heavily modified by the developers and released into the game in December 2013. The dungeon is one of the only sources of Tier 6 (Unbound) rings and has three very powerful untiered rings.
The dungeon portal has a guaranteed chance of dropping from the Avatar of the Forgotten King.