The key is for the door inside the cave.
Beyond the door is a small glade with two caracters, who come as a bit paranoid, and I believe not just because their buddy lies dead inside the cave.
Actually, they might be two "bandits" that the constable from Lumen Targ is looking for, there is a bounty posted in front of the prison with a bounty note. However I cannot confirm, because I'm still too low a level to kick their asses, and those two wipe my party every time I try to fight them. Also, the bounty note mentions two bandits, and those guys were three before their buddy got killed by the imps, so my guess might certainly be wrong.
I suppose coming back when I'm stronger will provide some solution.