From the introduction of Lost Mine of Phandelver, under the background section:

“A powerful force of orcs reinforced by evil mercenary wizards attacked Wave Echo Cave to seize its riches and magic treasures. Human wizards fought alongside their dwarf and gnome allies to defend the Forge of Spells, and the ensuing spell battle destroyed much of the cavern. Few survived the cave-ins and tremors, and the location of Wave Echo Cave was lost.”

Lost Mine of Phandelver, pg. 3

The calamity that befell Wave Echo Cave did not claim the lives of every combatant in the battle for the Forge of Spells. The text suggests there were survivors.

Without leaving the confines of the adventure module, there are three who could have been alive five centuries ago. It might be fun to run with that.

• Nezznar, The Black Spider: In the story, Nezznar is of an indeterminate age, but he could be of a venerable age five centuries later when the story takes place. Having escaped the Matriarchy, Nezznar sought to forge his own path in the world above as a mercenary wizard, which led him to that fateful day at Wave Echo Cave. His greatest opportunity to attain arcane power unraveled when the mine was destroyed, but after centuries of research and study, he believes he possesses the knowledge to repair the Forge of Spells. Now, if he could only find the place again.
• Agatha: Or perhaps it was Agatha who was brought evil to Wave Echo Cave, using her great beauty to control and corrupt the mercenary wizards under her command, killing hundreds in her quest to take the Forge of Spells for herself. When death came for Agatha on the site that, centuries later, would become the village of Conyberry, she would remain trapped on the material plane, cursed to remain a woeful spirit dwelling among the living as her eternal punishment.
• Reidoth: I know what you’re thinking. The old druid is a human, so how could he possibly have been at Wave Echo Cave when it was destroyed over five centuries earlier in 951 DR? Lucky for us, the game designers did not provide a stat block for Reidoth, leaving a nice opening to transform Reidoth into something other than a squirrel. The old man is not just any druid wandering the hinterlands of the Neverwinter Wood and the Sword Mountains. He is among the oldest and wisest of his calling, and through the primal magic he wields, he has a timeless body that ages him only 10 years for every century that passes. Perhaps before he found his true calling, he was a young apprentice wizard at the Forge of Spells. Barely surviving the battle, he wandered the hills and valleys of the Sword Mountains until he found himself under the eaves of the Neverwinter Wood where he found his true calling.

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