Tidalcore

The Tidalcore is a rare crafting reagent necessary for many high end Battle for Azeroth profession recipes.

Source
As of Mythic+ season 2, this item drops from the last boss in Heroic (not a guaranteed drop) and Mythic dungeons. It replaced Hydrocore which was the rare reagent drop in season 1. All crafting professions have the ability to convert Tidalcores into Hydrocores.