It is normal to miss things in an RPG the first time through. Even if you followed a
walkthrough /
guide (best used just when you are stuck, or skimmed over sections you have done already so see if you missed anything significant), there are quests that are mutually exclusive, and sometimes different quest paths are available based on earlier decisions / quests.
For Vigor Mortis, if you follow the river west and a little south of the lake, the mine camp you need is to the south. North of that river branch is the fortune teller and a bunch of Black Ring members. As you enter the camp there an NPC will call out to you and explain what happened, and the mine entrance is at the far back (west).
Even if you miss this, your necromancer will give you a crystal skull once you have the battle tower.
The 3 talismans, and the place to sacrifice them to get the forth, are all in the derelict tunnels:
Talismans.
Level 15 sounds about right for the end of Broken Valley. I was just about to level 16 at the end of Maxos temple. When I realized your first encounter on sentinel island you can mindread to go up a level, I reloaded to head to the fjords first to level (no sense wasting such a mindread when I only needed 3k experience to level). I kept going, and made it to level 18 in the fjords. The last 4k experience I needed there were only level 21 imps left, which were difficult, but I had a much easier time after I switched my creature from a warrior head (level 6 Rush Attack / Fatality) to a ranger head (level 2 Fireball); I didn't have a mage head (Magic Missile / Magic Blast) at the time.
