With Flickering Heart Quest - Is the Succubus guilty?

Solution 1:

If you buy the surgical tools from the plant vendor just outside Vergen's inner gate and take them with you when you examine the body in the crypts, you'll be able to extract a metal fragment from the corpse's arm.

It appears to be part of a sword, and Succubi don't kill people with swords. But without anything to compare to, Geralt doesn't have anyone to place the blame on. Take the metal shard to Iorveth, and show it to him when you accuse Ele'yas of murdering the young men. Iorveth will compare the metal shard Geralt found to one of Ele'yas' blades and find it a perfect match.

There's your evidence. Now go back and rescue Dandelion!