A crystal that has the ability to absorb spells and use the spells that get absorbed. The spell needs to come into contact with the crystal.

For example: If fire touches the crystal, the crystal will turn red and be able to use that fire. Absorbing the spell doesn't take up an action. For each new spell absorbed, roll for new spell slots.

There are three ways this crystal can get rid of its current spell:

1 - Using the crystal (too much)
The crystal can't keep the spell forever. 2d6 to determine the number of spell slots. And whatever the spell level is (and the creature using it), that's how many points are cut off.

Example: Level 3 spell - spell slots of 5 = 2 more times left (3-5=2). And when the crystal's spell slots turn to zero, they can no longer use the spell they absorbed and they can now absorb a new spell.

2 - Ejecting the spell
This does cost an action. Ejecting the spell releases the spell that is currently being used (like an explosion). And the explosion damage depends on how many spell slots the crystal has left.
Spell Slots left:

• Spell Slots: 1-3          Damage: 1d4          Range: 10ft
• Spell Slots: 4-6          Damage: 1d6          Range: 15ft
• Spell Slots: 7-9          Damage: 1d8          Range: 20ft
• Spell Slots: 9-10        Damage: 1d10        Range: 25ft
• Spell Slots: 11-12      Damage: 2d8          Range: 30ft

3 - Being Destroyed
Being Destroyed has the same effect as ejecting the spell. But the damage is reduced by 4. And the range is reduced by 5ft.