Acererak's Blackstone

You use magic to merge an ioun stone of absorption or ioun stone of greater absorption with a black gemstone into a single object. When complete, the blackstone is able to absorb magical energy in the following ways:

• Spells directed at the blackstone itself are instantly absorbed.
• If the blackstone enters the area of effect of an active spell the spell ends and the magic is absorbed by the blackstone.
• If the blackstone is within the area of effect of a spell as it is cast, it absorbs the spell and negates its effect.
• If the blackstone is touched to the surface of a magically created barrier such as a wall of force, shield, or prismatic wall the magic is absorbed and the spell ends.
• If the blackstone is touched to a creature or object affected by a spell the magic is absorbed and cancelled.
• The blackstone does not affect permanent magic items, but it can absorb spells cast from magic items.

The blackstone can safely absorb a number of levels of spells that is the same as the ioun stone that was used to create it. If the blackstone absorbs spell levels that exceed this limit, it explodes in a wave of raw magical energy. All creatures within 60 feet of the blackstone when it explodes must make a successful DC 20 Dexterity saving throw or take 1d8 points of force damage per level of magic absorbed by the blackstone. Creatures succeeding on their saving throw take half damage. A creature holding the blackstone has disadvantage on their saving throw, and takes full damage on a success and double damage on a failure.