The Weakstone was half of a Thran powerstone that had been cracked by Dyfed to open a permanent portal between Dominaria and Phyrexia. The stone, along with the other half, the Mightstone, was then covertly placed inside Glacian by Yawgmoth to poison him. The stones absorbed Glacian's essence, granting them his mastery of artifacts, with the Mightstone being able to empower artifacts and the Weakstone to deactivate them. After Yawgmoth's treachery was discovered, the two stones were used by Glacian's widow Rebbec to seal the portal they had been used to open.
5000 years later, the stones were rediscovered when the brothers Urza and Mishra accompanied Tocasia to the ruins of Halcyon to excavate the Caves of Koilos. The two apprentices discovered the pedestal on which the two stones were placed and removed them: Urza took the Mightstone, Mishra the Weakstone. In doing so, they had accidentally reopened the gate to Phyrexia.
However, the brothers were unaware of this as they fought first with guardian golems that they had awoken, then with each other. The two were each jealous of the other's stone. Their rivalry erupted into violence that their master Tocasia tried to stop, resulting in her death. Following this, Mishra fled to the desert, taking the Weakstone with him. He fell into the hands of the Fallaji, who enslaved him and took the stone from him. Mishra was later freed by the nomad chieftain's son, Hajar, who knew him from Tocasia's camp, but the stone was not returned to him until later when a dragon engine attacked the Fallaji. Mishra and Hajar used the confusion to retrieve the stone so it could be used to stop the beast, but instead Mishra found that he could control it. Using his new weapon, Mishra aided the Fallaji in expanding their power across Terisiare, beginning the Brothers' War.
Throughout the war, Mishra retained the weakstone, which he used to halt his brother's strongest creations. It was with him when the two brothers faced each other for the final time in Argoth, where Urza used the Golgothian Sylex to eradicate Mishra and his legions. However, while Mishra seemed to die, Urza instead ascended to planeswalkerhood. As this happened, the Mightstone and the Weakstone fused with him, becoming his eyes.
The two stones remained inside Urza for the rest of his life, aiding in his preparations for the coming Phyrexian Invasion. Prior to his death, the only time they were removed was in Serra's Realm, where its mad guardian Radiant tore the stones from Urza's eyes. This would have killed Urza, but Radiant then tried to combine the stones, which only resulted in causing a massive explosion that destroyed the angel and restored Urza.
The two stones were removed one last time following Yawgmoth's invasion of Dominaria. In order to defeat him, the Legacy had to be completed through Gerrard Capashen combining the stones inside Karn. Gerrard plucked out the stones, killing Urza, then pushed them together inside Karn's chest, triggering a massive burst of mana that drove Yawgmoth from Dominaria. This also caused Karn to transform into planeswalker, in whom the stones merged, granting Karn the accumulated knowledge of both Glacian and Urza.