The new Dual Talent Specialization Trainer added in Phase 3 of Season of Discovery known as «Grendag» can be found at the edge of Steamwheedle Port in Tanaris. For 50 gold, you can learn a dual talent specialization, a feature often requested in WoW Classic!
Edit: After today's edits and combat bugs, Blizzard has changed this location. Grendag will now be found in Orgrimmar and Ironforge, after the NPC is reintroduced to the game.
Players on Season of Discovery are reporting that purchasing the dual talent specialization (50 gold) from Grendag Brightbeard while in combat will result in losing the 50 gold, but not learning dual talent! If you are interested in buying this, be sure you aren't in combat or doing PvP!
When Blizzard updates players on the issue or fixes it, we will update this post with the latest information.
Edit: Blizzard has removed the Grendag from the game.
Edit: Blizzard shared a Blue Post explaining that Grendag was removed.
Edit: Blizzard shared on Twitter that Grendag will return tomorrow in a different location.
Edit: In the Patch Notes for Phase 3, Blizzard states Grendag will be in Ironforge and Orgrimmar.