The three amulets in the "derelict tunnels"? if so it's that stone statue right next to a corpse with some flowers and other vegetation near it.

Yes, it's need to finish this quest. I don't really understood where it is .What is the nearest waypaoint to it?

near a intersection in the "castle" part of that dungeon near a storage and a staircase plus a pressure plate that'll teleport you the first time you step on it, it's right next to a wall next to a doorway/archway and lit up, there's a black ring corpse next to it as well with a book nearby and it looks like a stone statue with flowers placed nearby it, room is also lit up a bit, hard to describe.

It's impossible to complete it because there is no conversation option when I try to inform Jedediah that Orobas is dead.

Landed on the platform outside and talked to everyone, finally talked to the "leader" and accepted the quest, this shouldn't matter but I then did some other stuff and after a while I entered High Rock or what the village name was and explored, found the key in the traders house and took the drudane plant, stored all but one of them in my tower's storage, this shouldn't really matter either, and arrived at the training center for the initiate dragon slayers, talked to everyone again and used mind-read to get a alternate way to get the chalice, did the quest related to the prisoner there but the chalice should be all that's needed, went back to the ruined fortress or what it's supposed to be and talked to the leader again and had him perform the ritual, entered the tunnel after talking to him again, probably doesn't matter either, I was just checking for additional dialogue, and explored the small room with the skeleton in it, got the cutscene and solved the chests riddle, which shouldn't matter either as it's a separate thing but you'll want the reward from it so save/load and try until you get it as you only get one attempt to open it normally.

Went back topside and talked to the leader again, small cutscene plays out and after that the quest will be resolved.

THX! I was thinking it's outside of the dungeon.

How to finish quest "A hunting We Shall Go". In that quest i alredy got Hjalmar, Hallorn, Viper, Jagon. Who's missing? Thanks

After killing whatever his name is Beregar the necromancer, I didn't get a book for the necromancer in my tower, so now I can't upgrade it, anyone else encountered this?

Goblin shaman near/behind the quarry a short while from Viper's camp.

The real reward is in the chest down there (Dragon armor, crystal quality, albeit it doesn't really matter what type it is as it's about the skill/attribute bonuses with dragon items and they all give the same bonus amount but for different skills and either HP or Mana.) once you solve the riddle albeit it depends on your skill "set" if you find it useful or if you'll just sell it later.

Aside from that you get a nice XP bonus and the typical choice between more XP, gold or a item plus a discount from the merchant there.
(Merchants don't "respawn" or "restock" or what to call it so if you've already bought what you want it might not be that useful.)

Plus a special little scene just before finishing the quest.
(There's a youtube video of it a couple of pages back.)

I take it you mean the chest that belongs to the goblin in the library, near one of the walls there are two flags and a bookcase which you can interact with, use it and then choose to read "Goethe" and the wall will open, enter and try to open the chest and then go back to the elevator and up one floor to the torture cell, enter it and a undead monster will rise from the slab in the corner of the room, kill him and take the key he has on him, go back down and unlock the chest with this key to get the information you need in order to progress to the top of the tower.