The Rat Leader Guy kept doing that to me too... I realised that there's a second part to that assignment: You've gotta kill Raskalion (which is rather sad; I liked him much better than the Rat Guy).

I read somewhere in the board here that to be an assasin you have to give Scanthril T`Harks lockpicks and kill Freydies. Now he tells me to find a burrow in the hills and say "Scanthril". Is that burrow in the hills - Booger`s Prison? But I been there before and wiped it clean!
Do I get this dagger somehow? Or I should load a saved game? I dont want to do that.

Take the Assassin's Dagger to the Pawn Shop in Brimlock Roon and the character will be offered the role. The dagger is your reward from the rat at the Burned Out Hut for killing the renegade in the Stout Mines. You may be able to invoke it first for the Backstab trait if you are the proper class, but anyone converting to an assassin will gain the trait.

I believe him sending you to the boogre caves is merely a way of getting rid of you.
Scanthril has nothing to do with either the lockpicks nor killing Freyedies. After finishing his quest (finding Grundel and killing his spy in the stout mines) you should recieve the dagger. (Make sure there is room in your character's inventory.)

Does rat Scanthril actually give the assasin`s dagger? He must dislike me. He continues telling me that Grundel is dead & offers to go for the payment to their hideout in the burrow of the nothern hill. And even killing him gives me only the scull dagger. I dont have a patch installed, though. Maybe that`s a bug.

Ask him about another JOB - that should stop him trying to send you to the Boogres caves (just to get rid of you) and get you onto the real assassins quest.

Wyv