Rahvan's Curse - Act 2 Side Quest
Location: Garden of the Ancients + Azunite Catacombs
Closest Portal: Garden of the Ancients (South)
Objective: Obtain all 4 Champion Death Masks then defeat Letiso
Reward: Rahvan's Fist as well as other Loot + Gold
Rahvan's Curse is a Side Quest in Act 2 which begins by speaking with Champion Rahvan after you've found all 4 Champion's Death Masks from around the Garden of the Ancients. You will need to find 3 Champion's Death Masks in order to complete the Princess Evangeline Story Quest and advance your game forward; finding the fourth is optional and triggers this quest.
If you'd like to see a map location for every Champion's Death Mask I recommend you use my map screenshot below. Aside from finding all of the masks all you need to do for this quest is head into the Azunite Catacombs and defeat Letiso.
You can find Letiso in the bottom portion of the Azunite Catacombs; it's the same location you free Princess Evangeline at during the Princess Evangeline Story Quest. When you ride the elevator down there will already be a silver star on your map where you need to go.
There is a room on the southern end of the Azunite Catacombs which protrudes a bit from the main hallway that goes through. Along the eastern end of this room there will be a large symbol on the floor which should look familiar. Stand ontop of it and use Chant #75, the one Champion Rahvan gave you. This will open the door to the Lich Letiso.
In your big book of chants, the one we're looking for is the second before last and the text for the chant is 'Letiso'. Once you open the secret passage all that's left is to defeat Letiso. Return to Rahvan after you do this for your reward and to complete the quest.