This event is called Tirion’s Gambit and comes after The Hunter and the Prince. This is where you actually try to attack the Lich King’s heart and destroy it.
The quest before this one is Argent Aid. Highlord Tirion Fordring attempts to use the [Ashbringer] (famed sword in WoW lore) to destroy the Lich Kings heart (which he removed from his body so that he wouldn’t have any feelings or remorse - he’s a badass).