Yup it is covered in Book 2 of the Return of the Archwizards Trilogy, and as will points out is also summarised in the Players Guide to Faerun.
In the Year of Wild Magic ,Cormyr after having diplomatic talks break down with the Shade Enclave, and on discovery that Shade was using Shadow Blankets to melt the High Ice, went to war against Shade and the Phaerimm both.
The melting of the high ice was a problem because it was causing immense shifts in the weather. In order for Shade to turn the Anauroch back into the verdant land it was during the time of Netheril, they were causing massive floods, storms, and famine everywhere across Faerun..
The Allied forces of Cormyr, Sembia, Waterdeep, and the North (Silverymoon, some Uthgardt, Evermeet elves and the Dwarves) waited in Tilverton (which before the breakdown of talks with the Shade was actually being rebuilt by the Shadovar as a gesture of friendship), and a battle was fought there..
During the battle Tilverton was pretty much dissolved by miasmic clouds of shadow stuff that disintegrated pretty much anything they came into contact with, from people to buildings. This shadow stuff was a result, in part of shadow magic clashing with weave magic directly, as Vangherdahahst and Galaeron along with the giant Aris were using a new weapon to fight the shadovar. This weapon was a globe with moving glyphs that targeted the shadovars shadows, preventing them from escaping and then destroying them. By the time it was over the Shadovar had been beaten, but at the cost of Tilverton and most of the Alliance army.
Shortly thereafter the Phaerimm were discovered during a meeting of alliance leaders, where they manipulated the leader of shadowdale and his two subordinates to try and sway talkss away from aiding the elves of Evereska.
The Evereskans were trapped with nearly one hundred phaerimm inside the Shaeridimm, inside a Shadow shell the shades had erected, that was not only killing the Phaerimm by starving them of magic, but also destroying the mythal of Evereska that was keeping the surviving elves safe from direct attack.
When the Phaerimm at the meeting was eventually discovered by Galaeron and the Bedine Witch and Harper Ruha, it offered the alliance terms. He stated that if they did not help Evereska, effectively giving them the area, that the Phaerimm would destroy the Shadow Blankets melting the High Ice...
The alliance didn't agree, but neither did they disagree and with the matter hanging the Phaerimm stated that as a sign of faith to prove their offer was real, once the shadow shell around Evereska fell they would destroy three of the shadow blankets as proof they meant what they said.
Without much of an army left the alliance had little in the way of might to back up any threats, especially against the phaerimm and their armies of mind slaves, beholders and illithids... And also not while the potential for another attack from the Shade Enclave could occur at any time... As such Evereska for the time was pretty much left to stand or fall on its own for a time.
