Elminster

Elminster Aumar is a legendary wizard, originally from Shadowdale. A key bearing his symbol appeared on Diath's key ring while the group sailed from Port Nyanzaru to Waterdeep. A talking fish delivered a message from Elminster to Strix while the heroes sailed from Port Nyanzaru to Waterdeep. It warned her to keep an eye out for clues from Elminster (part of the Immortal Fortress storyline).

D&D Lore
Elminster Aumar is a legendary D&D NPC who has starred in many novels written by Ed Greenwood. Elminster has appeared in many RPG products, as well as a series of Dragon Magazine short stories in which he spends time with wizards from other settings, including Mordenkainen. Recently, Elminster was featured in Volo's Guide to Monsters as a "co-author" of the in-game version of the tome.

Immortal Fortress
Elminster seems to be a key component in the current "Immortal Fortress" storyline, wherein people are receiving USB "keys" in the mail with cryptic clues on them. This story presumably is linked to the upcoming adventure that will take place after Tomb of Annihilation.