So I was doing the quest Tolled to Rest and needed to escort Oskar. I started escorting him and saw the cyclops and the drake fighting it out. We got further into the cenotaph and I found a ballista on the battlements. I decided to fire a cheeky shot at the drake since I hadnt done it yet and wound up in a long fight with said drake. I finally killed the drake, however now Oskar is nowhere to be found. I still have the quest and it doesnt say that I failed the quest and I just have a yellow area marked on my map which is where he is supposed to go to, however he isnt there or in the surrounding area.

I have no way of completing this quest.

Is there any way to abandon the quest or reset it or something? I cant load a save or anything anymore, really stuck on what to do.

Edit*

Okay, so for anyone who finds this post looks for an answer, good news is I have one for you (hoepfully). So for starters if you have lost Oskar, check back by where you first meet him. If he isnt there leave and come back or rest for a few days or something, maybe even try reloading the game and he should be there. Worst case scenario his dead body will be in the Vernworth morgue (Vermund Charnel House) and you can revive him using a wakestone, although I never had to do this.

Secondly, if you no longer have the book because you either sold it, lost it or used it. You can get another one. It is called "Nation's Death Knell" and it is simply one of those books that lets you cast a spell. In this case it is Meteoron. You can find more, its just its a very rare one to find. There might be other places to find one but there is one you can get very early. In the cave right near the start of the game called "Waterfall Cave" which is here on the map https://imgur.com/FeJHzq5 you can go inside and keep going till you start seeing red Saurian. There will eventually be a room with a bunch of books after a room with a bunch of stuff you can mine. There are some books scattered on the floor but there is a chest and in the chest is the book "Nation's Death Knell". You will have to fight one of those undead litch boss things, so be prepared. A silence spell on you or a pawn will help a lot. Once you have that book you can go back and hand it over to Oskar. Be warned that that cave is massive and very windy. You might have to do a lot of exploring to get there, but trust me its in there.