This is considered general knowledge, details of which however may require certain knowledge checks if applied in-game. The results of this event will generally mean that you won’t stumble upon any legendary artifacts any time soon.
The Spellplague is a malady that struck Realmspace on 29th Tarsakh, 1385 DR and was caused by Mystra’s assassination at the hands of Cyric and Shar. With the goddess’s death, the Weave, the universal structure of arcane forces, convulsed. It continued for a decade, leading to the Wailing Years, during which time arcane magic ceased to function and the planet of Toril was transformed.
“The white queen is troubled but can’t say why. The black queen hates the white and gives the assassin a black coat. The assassin steals upon the white queen. She can’t see him gliding through the shadows. The sword screams. The white queen falls. Her city falls. Stones fall in the cavern to crush the soothsayer. The tree burns and thrashes in agony. Branches break. Branches twist and grow together…” — from a vision by the diviner Yaphyll in Undead by Richard Lee Byers
In her vision, Yaphyll refered to Mystra as the white queen and to Shar, goddess of the night, as the black queen. Cyric, god of murder, is the assassin, Savras, the god of divination, is the soothsayer, and the city, cavern and tree symbolize the ordered structures of magic crumbling into chaos.
The break-down of the Weave could be felt by all wizards across Faerûn.
Effects on arcane magic With the structure of magic out of balance, even with great effort many spells failed or produced unreliable results. Most permanent magical items, such as artifacts, were left intact at the end of the Spellplague and charged magical items were either destroyed, warped, or simply ceased to function. Items with dormant magical qualities, which were manually activated, were largely unaffected by the Spellplague.
Other effects The Spellplague affected the landscape, making the ground rumble and heave up and down like the surface of the sea. Curtains of blue flame swept the landscape reshaping the land by cutting crevasses or lifting and sculpting the plain into hills and ridges. Waves of azure fires appeared everywhere, killing everything it touched. A surge in the use of technology, divine magic, and worship of the Deities of Invention and Knowledge. Political upheaval across the lands that resulted in many attempted coups, invasions, and uprisings. Especially against governments, and nations that had relied heavily on arcane magic for defense or controlling their population.