Received this quest at the start of the Chasm map but I can't find it.
It shows a s a separate quest from the Spellplague Caverns
There were several players asking the same in chat but no response
It is an actual dungeon.
The queue system refers to it as "Spellplague Caverns"
It's located not far from the quest giver, the very end of the map, slightly on the right - far north of the final area in the chasm iirc.
And yea pretty dumb the quest trail doesn't function for what's arguably the best part of this game. (Enjoying dungeons without suffering the horribly broken queue system).
Also that quest, like most quests related to dungeons - is broken. If you want to cheese it, you can simply queue for the dungeon, and leave immediately once inside - the quest will auto complete (and a few ppl may be upset someone left the group).