The Sylex Blast was the climactic and traumatic explosion that ended the Brothers' War. After realizing that his brother had become a Phyrexian and noticing the unseen influence of Gix on the entire war, Urza used his newly found magical skills to activate the Golgothian Sylex. The outburst of energy from this instantly vaporized all but a fraction of Argoth, and sent a massive shock wave across Terisiare that reached as far as the Ronom Glacier. It also destroyed the tectonic plate beneath Argoth. Terrible as this was, it succeded in Urza's aim to stop the Phyrexian influence on Dominaria.
While both Urza and Mishra were disintegrated by the blast, the Mightstone and the Weakstone united to resurrect Urza as a planeswalker. Realizing the damage both he and his brother had inflicted on the world. Urza rescued his apprentice Tawnos, told him to tell the story of what happened and departed Dominaria.
So terrible was the blast that Dominaria began to form the Shard of the Twelve Worlds, which trapped twelve planes in a bubble from the rest of the multiverse and stopped any transplanar magic or technology from reaching them. While this kept Urza out, it also helped protect the devastated Dominaria from Phyrexia, preventing them from entering for almost 3,000 years, when the Shard finally fell.
Hundreds of years later, the cataclysmic effects of the Sylex Blast were still felt by Dominaria. Time Rifts, caused by the Sylex Blast amongst other monumental catastrophes, were tearing reality apart. These rifts were only healed with the sacrifices of a handful of planeswalkers, furthering the death toll claimed by the Sylex Blast.