An entire region or so will be in red on your map. you'll see a sword symbol in the middle with a timer. That's a Helltide event. Random chests will be in the area that can be opened with the cinders (a temporary currency that disappears after the event). you run around and kill enemies, look for these special chests and open them for loot. Chest cost 75, 100, 125, 150 and the best 175 cinders. The 175 has guaranteed legendaries. The catch is that if you die you lose half of your cinders and the enemies in Helltide are higher leveled, there's lots of them and they are aggressive. Periodically you'll see meteorites fall on an area hurting you and other players. So its a careful balance of being greedy and looking and going for the higher levelled chests versus dying and losing half of your cinders. It's a solid event to level up as well because enemy density is high. You should be running them often.