Mightstone

The Mightstone was half of a Thran powerstone that played in major part in the Brothers' War on Dominaria. Its counterpart was the Weakstone.

Dyfed
The Mightstone was an ancient Thran crystal capable of empowering artifacts. It was created when Dyfed cracked a large powerstone in half to establish a permanent portal between Dominaria and Phyrexia. Afterwards, Yawgmoth secretly placed the two halves (the Mightstone and the Weakstone) inside Glacian to worsen his case of phthisis. While inside him, the two halves absorbed Glacian's essence, empowering them with his knowledge of artifice. After Glacian's death, his wife Rebbec discovered Yawgmoth's evil nature and so combined the two halves to close the portal Dyfed had created, sealing him in Phyrexia.

Urza
Following the destruction of Halcyon in the Thran Civil War, the two stones remained in the Caves of Koilos for approximately 5000 years, until the ruins of the Thran capital were excavated by Tocasia and her two apprentices, the brothers Urza and Mishra. As they excavated the Hall of Tagsin, the two brothers uncovered the pedestal where the two stones were kept together. Urza took the Mightstone and Mishra the Weakstone, unwittingly reactivating the gate to Phyrexia. They also accidentally awoke the golems that guarded the vault, who then promptly attacked the brothers and their master. Urza tried to use his stone to stop them, but found that he had only made them stronger, the creatures only being stopped when Mishra used his stone against them.

As the excavation work wore on, the brothers became increasingly jealous of each other, each wanting to possess the other's stone. This came to a head back in the encampment, where they argued and became violent, using the stones against each other before accidentally killing Tocasia when she tried to calm them. After their master's death, the two brothers separated and began building armies, beginning the Brothers' War. During this war, Urza retained possession of the Mightstone, using it to bolster his legions.

At the climax of the long war, Urza used the Golgothian Sylex to obliterate his brother and his army. The blast also destroyed Urza's forces and levelled Argoth, but Urza himself survived thanks to his spark triggering. As he became a planeswalker, the Mightstone and the Weakstone merged with him, becoming his eyes.

The two crystals remained embedded in Urza's head throughout his life as a planeswalker, still aiding in his creation of artifacts. The only occasion prior to Urza's death in which they were removed was when Urza returned to Serra's Realm to use the collapsing plane to power Weatherlight's planeshifting engine. He discovered that Serra had been forced to flee her realm after the Phyrexians attacked it and brought black mana corruption in their wake. Serra's replacement was the angel Radiant, who held Urza responsible for the devastation afflicting her plane. She trapped Urza inside Serra's Sanctum and attacked him, plucking the two stones from his eyes. This would have killed Urza, but Radiant then foolishly put the two stones together, triggering an explosion that killed the angel and restored Urza.

Karn
The stones were removed one last time following Yawgmoth's return to Dominaria. The only way to stop him was to complete the Legacy, so Gerrard Capashen had to remove the stones from Urza's eyes and combine them inside Karn's chest. He did this, triggering a burst of energy that drove Yawgmoth from Dominaria while killing Gerrard and causing Karn to become a planeswalker. The combined stones merged with Karn, granting him the knowledge of both Glacian and Urza.