Niflheim Purgatory is a 6-man instance where you fight 4 Revenant MVP (RMVP) bosses. Everyone in the group receives drops.
Once you reach base level 80 Niflheim Purgatory will unlock in the Instance menu.
Location of Niflheim Purgatory in the Instance menu
You may queue up for 5 stages across two difficulties, either Warrior or Elite. Each stage offers different bosses but you can only complete one of these stages per week so choose the one with bosses that drops the most valuable goods and hope you get lucky!
If you wipe to a boss you cannot challenge them again until next week. Make sure you’re in a strong group before you challenge these because some have crazy mechanics. These days many people can outright one shot these bosses so most groups can probably steamroll this content.
Revenant MVP bosses are undead so they are extremely weak to the fire and holy elements. This is a great way to take advantage of priest's Aspersio spell which converts party members damage to Holy element.
Avoid things that change the monsters element such as the Tidal Shoes set bonus since this may drop party damage significantly, especially since you're not geared for this as a new player and the rest of your parties damage cannot take advantage of the elemental change.