Eidolith

Eidolith is a type of crystal from Final Fantasy XIII and Final Fantasy XIII-2. They are bestowed by fal'Cie upon people, called l'Cie, which allow to use magic and summon Eidolons.

Eidolons
''These mystical entities reveal themselves before only a select few l'Cie. It is said that they are saviors, come to rescue helpless l'Cie who find themselves bound to a Focus against their will. If this is true, they offer a salvation few indeed would seek willingly: without exception, Eidolons attack the l'Cie whose presence they grace.''

''There have been no l'Cie in Cocoon for centuries, and as a result, no way of determining the truth behind tales of these beings. To the citizens of Cocoon, they remain the stuff of bedtime stories.''

Final Fantasy XIII
When Lightning, Sazh, Snow, Vanille, and Hope are branded l'Cie by fal'Cie Anima within the Pulse Vestige on Cocoon, crystals—eidoliths—were planted within them allowing them to use magic which normally is limited to feral creatures.

When Snow is surrounded by PSICOM troops, his l'Cie brand activates and forms twin sisters Shiva to materialize from the eidolith within him. The sisters kill soldiers and turn against Snow who proves his strength to them allowing him to summon them in times of need.