Step 5, Option A: Kill the succubus
If you decide to kill the succubus, the fight will start immediately. You have to look out, as the enemy has a few tricks up her sleeve as well. Use Quen and try not to attack when she has it active herself.
Step 6: Talk to Ele'yas
Return to Ele'yas (M15, 6) and tell him that you killed the one responsible for the mysterious murders. In return you will receive orens and the Angivare sword.
Prize: 200 XP
Step 5, Option B: Talk with Ele'yas
You will learn that someone has fallen in love with the succubus and is killing her lovers - supposedly it's Ele'yas. You can either help the woman or don't believe her - in the later case you will be moved to Step 5, Option A. If you decide to dig deeper, return Ele'yas (M15, 6) - you still will be able to change your mind.
Step 6: Tell Iorweth you accuse Ele'yas or kill the Succubus
Remember that you still can return to the succubus and kill her (Step 5, Option A). If you decide to tell Iorweth about your suspicions, you can find him in the upper part of the city (M15, 18).
Notice! If you don't have any proof of Ele'yas' guilt, you need to return to the dwarven catacombs (M13, 14) and take out the Metal fragment form the corpse (M18, 3) using the Surgical tools - they can be bought from Felicia Cori (M15, 17).
Show Iorweth the fragment. Unfortunately Ele'yas will manage to escape before the elf manages to get him.
Step 7: Talk with the succubus
Head back to the succubus (M13, 2). In the burden village you will come across Ele'yas, who will immediately attack you. The elf fights with two swords, so you have to be careful. Try to stun him often with Yrden. Your enemy fights like a berserker and keeps charging at you at all times, so remember about having Quen active and to dodge often.
Once you get rid of him, go into the succubus' quarters and talk with her. You can either agree for a moment of pleasure or refuse. If you agree, the screen will be blurry for a moment after you finish (as the demon has sucked out your energy).
Prize: 200 XP