Helltide is part of the Diablo 4 end-game.
<bluepost>While roaming Sanctuary on foot or atop your trusty steed, be aware of your surroundings, for you might find yourself caught in a Helltide. This is a new region-wide event that becomes available to heroes only once they have reached World Tier Three: Nightmare difficulty. In a Helltide, the servants of Lilith are empowered, having their difficulty increased, but dropping loot worthy of the danger. Her mightiest minions are also out in greater force, and the very ground of Sanctuary as you know it has been morphed by the surge in demonic activity.
As you cull the monstrosities inhabiting a Helltide, they’ll potentially drop Cinders, a new currency that can be spent to open Helltide Chests scattered throughout the respective region. These chests boast bountiful boons exclusive to a singular item slot, such as Torso, Legs, or Two-Handed Weapon. But be wary in hoarding your Cinders—should you fall in battle, they’ll drop and must be reclaimed.<bluepost>