Fractals of the Mists are level 80, 5 man “dungeon” levels. There are an array of different Fractals that a player can embark on, each with it’s own style, story, and boss fights.
There are 4 tiers of Fractals. Tier 1, which is the easiest Fractals to complete, and have no debuffs. Tier 4 is the hardest and can contain 3 debuffs known as “Mistlock Instabilities”.
The most prominent threat of higher level Fractals is Agony. Starting at Fractal 20, Agony is a condition that reduces healing and deals a large percentage of your character’s maximum health as damage. As the difficulty of the Fractals increase, so does the maximum damage that Agony inflicts. Starting at 10% and then capping at 180%. To counteract Agony, there are infusions called Agony Infusions (as expected), these can be equipped onto ascended gear, which then can reduce the Agony down to as small as 1% max health as damage. This makes Ascended gear vital to progressing through the fractals.