Eurydice and Orpheus - Act 4 Side Quest
Start Location: Upper City of Lost Souls
Quest Location: Cave of Whispers/Upper Daemon Camp
Objective: Obtain the Mirror of Psyche from The Bloated One in Cave of Whispers; then free Orpheus from the cyclops
You'll receive the Eurydice and Orpheus quest by speaking with Eurydice in the Upper City of Lost Souls, a town in Act 4. If you've forgotten how to reach this town you'll want to take the Plains of Judgement Portal, that'll put you right near the quest giver. This quest involves a few parts, first you'll need to go into the Cave of Whispers in the Upper Daemon Camp and acquire the Mirror of Psyche.
The location of the Upper Daemon Camp is shown in my map screenshot below, click to enlarge it if you need help finding this place. In the back of the Cave of Whispers you'll find an Albino Spider named The Bloated One guarding a chest. This boss is very difficult to defeat unless you have good gear, a strategy that may help you is listed below.
The Bloated One Boss Strategy: The Bloated One is a very difficult boss, definitely the hardest side quest boss you will face in the entire game. I strongly recommend you clear out all of the Albino Spiders around her prior to engaging because you'll need the room to kite when low on HP. When this boss summons little spiders it's recommended you kill all but 1. If there is 1 left the boss won't resummon them immediately and the single add won't be able to use the red lightning chain-link with any other adds.
Once you've defeated The Bloated One you'll be able to open the chest it was protecting and acquire the Mirror of Psyche. Return this mirror to Eurydice in the Upper City of Lost Souls to update your quest and unlock the gate next to Eurydice. Go through the gate and you'll find yourself in a new area 'Small Rocky Cave' - there isn't much to do here except defeat the two named cyclops in the back and free Orpheus.
The cyclops you'll face aren't anything unique - they have the same attack patterns as all other cyclops in the game. Just dodge their howls that stun you and they'll be pretty easy. Once they're dead speak with Orpheus to update your quest and then follow him back to Eurydice. Speak with Eurydice once they're reunited to complete the quest.