Scanthrill didn't give me the assassins dagger. He sent me to a cave to the north where they keep their loot. I also have a quest to kill a cyclops in the north, and to find the Aku idol. Do these take place in the booger cave.

You need to find Grundel's body and report back to Scanthril.

The Cyclops is in an area outside the Boogre Caves that you can only reach after going through them. There are two exits at the end of it...one of them goes to a restricted area where the Cyclops hangs out.

The Idol of Aku is in the Boogre Cave and is what will transform you back.

as you approached the burnt out hut did Scanthril say "Grundel is dead isnt he I knew it" if so ask him for your reward and he will give you the dagger. If not you still nedd to find Grundel.

NOw Sezerac maintains Grundel is the dead body with the writ. This makes sence as Grundel would need a writ to get into the mine.

However Scanthril will send you off in the direction of the Booger caves and there is a dead body on the island in the lake. Was he off to get some booger brew to bribe his way in.

I also found a dead body in the stout mines near were you meet Raskillion. May be he got into the mine but Rask smoked him first.

As I found all three before returning to Scanth I cannot tell you which one is correct as he gave me the dagger no problem

Scanthril want wants you to kill Raskalion after finding Grundel. When he's dead you'll recieve the dagger.
Good luck in the bugger cave.