Hades Treasury - Act 4 Side Quest
Start Location: Lower City of Lost Souls
Quest Location: Multiple Zones
Objective: Collect the 4 Crystals and open the treasury door
Hades Treasury is a Side Quest which begins by speaking with Aristonymus, a shade found in the Lower City of Lost Souls. You do not actually have to speak with Aristonymus to complete this quest - you can collect the 4 stones mentioned below then come here and place them in the pedestals without ever talking to him. However, speaking with him will begin the quest.
To reach this location easily you'll want to take the portal to Plains of Judgement, that'll put you in the town in the Upper City of Lost Souls. Exit south out of town and go down the stairs, you'll see the cave entrance that leads to Aristonymus right in front of you.
If you're having trouble locating the cave use my map screenshot below. Aristonymus will be inside the cave down a set of stairs; in the same room with him you'll set a group of 4 pedestals. You will need to collect the 4 stones mentioned below and place each of them into the pedestals to open the door. Once you enter the treasure room after the door is opened the quest will automatically complete.
Below you'll find the locations of all 4 stones that you need to open the vault's door.
Frost Stone: Thyia in the Lower Daemon War Camp. Right near Keuthonymos for An Inside Source Side Quest. Hard to miss this enemy.
Venom Stone: Canace in the Second Domain of the Tower of Judgement. She is found in the north-central portion of the map.
Flame Stone: Metriche in Philos Plateau. Go up the wooden walk ways to get to him.
Soul Stone: The Night Mistress in Hades Palace. She is found outside the door that leads to Hades, quite difficult to miss.
Thyia of the Stygian Frosts - Frost Stone
Location: Lower Daemon War Camp
The first stone you will come across is dropped by Thyia in the Lower Daemon War Camp. As aforementioned it is almost impossible to miss this enemy since she is nearby both the An Inside Source Side Quest as well as Admetus Among the Dead Side Quest. You will have to run past her a few times to complete An Inside Source.
Canace the Serpent Queen - Venom Stone
Location: Tower of Judgement - Second Domain
You can find Canace the Serpent Queen in the north-central portion of the Second Domain, an area in the Tower of Judgement. It's hard to miss this boss since she is in the middle of the map but it is possible. Be careful when you kill her near the holes in the ground because it is possible that your Venom Stone rolls off the edge and plumits into nothingness. If this happens just log out and back in then defeat her a second time.
Metriche - Flame Stone
Location: Philos Plateau
Without a doubt the hardest of the 4 stones to find is the Flame Stone. Metriche is the enemy who holds it and you can find him in the Philos Plateau which is northwest of the Fields of the Diadochi area. You will reach this location shortly after leaving Elysium and traveling north on your way to Hades Palace.
In order to reach Metriche you will need to climb up the wooden walkways that connect the Philos Plateau and the Fields of the Diadochi area. One of the main reasons this stone is so hard to get is because it isn't self-evident that you can walk on these platforms the first time you come here.
The Night Mistress - Soul Stone
Location: Polis Daemonai
Last but not least you will get the final stone, the Soul Stone, from The Night Mistress in Hades Palace. This boss guards one of the final doors that leads to Hades and is impossible to miss.
Finishing The Quest
Once you have obtained all 4 of the required stones, return to the Lower City of Lost Souls and place each of them into the pedestals pictured above to open the door. After you've done this all that is left to complete the quest is to go into the treasury. The moment you enter the treasury we'll be all done and the quest will complete.