Geralt receives Echoes of Yesterday from Yaevinn at the beginning of Chapter III, if the witcher happens to be in the Druids' grove in the swamp. The elf asks Geralt to locate his men who appear to have gone missing in the sewers of the Trade Quarter.
Walkthrough
Find Yaevinn in the swamp and get the quest, then proceed to the Trade Quarter (Triss' house) using the teleporter in the Mage's tower. From there, head to the sewers.
After killing the ten or so drowners en route, you will find a dwarf named Ren Grouver and two elves standing around having a conversation about a skirmish. Talk to Ren. He will tell you about their most recent encounter with a monster in the elven ruins and ask you to get rid it. The beast is not only blocking his path and but has killed some of his compatriots. He describes one of the unfortunate as having simply "exploded".
Take the mission and proceed into the ruins through the tunnel which can be seen through a large opening in the sewer wall. When you arrive at your final destination, you should see a cut scene featuring a succubus-like creature. Enter the room and kill it — it is a bruxa (vampire).
When you are done, Ren will enter the hall and thank you for your help. At this point Geralt mentions that the fight was "worth more than [Ren could possibly] know", which is strange as the actual battle is a breeze, but I digress. Next, Yaevinn arrives via the Elven Ruins teleporter, thanks you, tells you to forget what you have just seen here and gives you your reward. That is it, all done.
Notes
- If you have completed A Posh Reception first, this quest and Six Feet Under are mutually exclusive. If you finish A Posh Reception and Six Feet Under, if you try to make your way anywhere at all, a child will stop you and give to you the Gold Rush quest. The relationship between A Posh Reception, Six Feet Under and this quest may be tied to the Identity quest and talking to Thaler during A Posh Reception, so if the dialogue line about Geralt's motivations is not pursued, it may postpone the event trigger. That has not been tested, however. See the Gold Rush quest for more details on the relationships between these quests.
- Even if you ignore the child and go to the swamp anyway, you will still find Yaevinn but he will not give you the quest because narratively he is really at the Vivaldi's bank in the basement.
- It is advisable to do the first few quests in Chapter III in the following order:
- first, acquire this quest from Yaevinn, then talk to Siegfried and get Six Feet Under, and undertake them in any order, (previous editions may prefer you to complete Six Feet Under first, but not in EE.)
- you could sneak in Dandelion's Lute here, and every other quest in Chapter III with the exception of the two below, Beauty and the Beast, and anything involving Velerad.
- A Posh Reception, and finally
- Gold Rush.
- If you fail (failure is such a harsh way to put it!) to get the Echoes of Yesterday quest, it is not the end of the world, or even of consequence to the game narrative particularly. You will lose the 6500 quest XP and the XP for killing the bruxa, but with some work these XP can be made up by killing the endless supply of alps also found in the ruins. The Alps are present before this quest and Gold Rush, but do not appear again until you have visited the bank again after completing Gold Rush. This seems to reset the spawning of the Alps. The Elven Ruins teleporter will not be immediately activated after this quest, but it does get activated after completing Gold Rush.
- Upon accepting the quest from Yaevinn in the Druids' grove, he will tell you a story about Lara Dorren and mention Feainnewedd now grows where she "breathed her last." This conversation does not unlock Feainnewedd in the journal.
- Completing this quest causes the dwarven blacksmith to start doing business with Geralt.
- Completing this quest causes the Order armourer to stop doing business with Geralt.
- If you have problem defeating the Bruxa on hard mode, bring some Devil's Puffball bombs as they work quite well against vampires.
- Do not use the teleporter to go to the elven ruins while this quest is in your journal if you have yet to find Yaevinn's men in the sewers. It can mess up the quest. Using the teleporter to travel to the ruins without first speaking to Ren causes the bruxa to just repeatedly use her attack on the corpse, ignoring you. You can not target her or interact with her in any way, and her nameplate is blue. If you try to leave into the sewers, it shows the brief cutscene of her attacking the corpse as it would if you entered from that direction and she then starts attacking you.
However, when you kill her the normal cutscene of Yaevinn and his men approaching does not start, leaving you stuck.After killing her, you can go back to the sewers to talk to Ren. Assuming that this would be your first encounter with Ren, all the conversations with him and even Yaevinn will take place right there instead of the ruins, so it just looks weird.
Phases
The Elven Temple
Yaevinn asked me to find his men. Apparently, they descended into the sewers to find an old elven temple. He also asked me to recover all the artefacts related to Lara I can find. Yaevinn asked me to find his men in the sewers of Vizima.
Evil
I met Yaevinn's men. Just as he said, they tried to get inside the temple, but found something living there, something evil and powerful. Only two elves and one dwarf survived. I should find out what happened to the others. I should find out what killed Yaevinn's men.
The Vampire
the bruxa causing all the trouble
The temple was home to a vampire, one powerful enough to kill most of Yaevinn's men. It was a tough fight, but I slew him. (editorial — oh really? and wasn't it a her?) The elf should reward me generously. I slew the vampire who killed Yaevinn's men. The elf should reward me generously. (2000 XP)
Yaevinn
Although I found no mementos, Grouver thanked me. It's all very strange... Why did the Scoia'tael need to get to the elven ruins? Grouver also said Yaevinn would pay me for completing the job. We found no artefacts. Why did the Scoia'tael want to get to the elven ruins? I need to speak with Yaevinn.
Elven Ruins
I sense Yaevinn hasn't told me something. Why did he want to get to the elven ruins so badly? There's nothing here... I brought Yaevinn to the elven ruins. (300 
+ 4500 XP)
