Seven Sigils War

The Seven Sigils War was a conflict between the wizards of Netheril against extra planar Immortals that they'd accidentally allowed to access Toril.

Location
The conflict began in the at the Netherese domain of Rdiuz, which was located along the Gods' Legion Mountains (currently known as the Desertsmouth Mountains). The Netherese had built a mighty floating citadel they named Meigg and were preparing to send troops into the Outer Planes through a massive portal called Cats Gate, seeking to conquer communities in the Outlands.

Causes
The portal turned out to be insecure, and allowed extra powerful planar Immortals from the Outer Planes to enter Toril, with initially devastating results to the Netherese.

Battle
Desperately seeking quick ways to regain the upper hand, the Netherese wizards set upon the idea of using natural enemies of Immortals, the ancient Primordials of the Elemental Chaos. Knowing that the Anchor of Chaos in The Monument of the Ancients was actively preventing the Elemental Chaos from boiling into Toril through a dimensional weakness, they went there to deliberately sabotage the Anchor, which allowed multiple primordials to enter Toril for the first time in long ages.

Aftermath
In the, after over a century of bitter conflict, the Netherese managed to banish most all of the extra planar entities that they had accidentally and deliberately summoned into Toril. Notably, one primordial, Maram of the Great Spear, escaped their attentions and fled into the wilderness.