I'm in Chapter III, Roche's path, trying to do the From a Bygone Era quest. I found the will-o-wisp, and when I talk to him, my only dialogue option is "See you", and it says I need to find more information on Malget's Notes, but when I take them to Bras of Ban Ard, I have no dialogue options having anything to do with Malget's Notes, and the quest tracker stays on top of the will-o-wisp. Is my game glitched, or am I just missing something?
I'll load a save and see what I can do. I think you're just missing some quest-essential crap.
Check Dandelion's entry in your journal in the meantime - perhaps you've overlooked something?
Edit: Okay, I did a little snooping around in a save of mine and found some stuff out:
Firstly, I do not recall the "Will-O-Wisp" being in this quest at all (or even remotely related).
Once Bras of Ban Ard deciphers the notes, you take them down below the city to the sewers (which are right next to The Mighty Numa, according to the wiki) and then speak to the guardian door. I believe the Will-o-wisp of which you speak is almost directly above (in terms of altitude) the guardian door. You need to go beneath
the city. Once inside, the quest progresses as usual, with some puzzling puzzles and some combative combat.
Hope that all helps.