I have completed Scanthril's quest to kill Raskellion. I went back to claim the dager from him but he tells me to go to a cave north of him, is that Bogger ville?

Storm

he will give you the dagger and only after that will tell you to go somewhere north.Unfortunately i can't help you with this hideout (i posted a question about it too,we'll se if someone answers to that).

I'm a bit hazzy on this, but as I recall the assasin's dagger is cursed, so I sold it. I think you should get it
after killing the guy in the Stout Mines, if you got the quest to do so. If you kill him before being asked to do
so you may not get the dagger.
As far as I know there are no "caves to the north" where you go to get your reward - he's just sending you to
the Boogre Mines hoping you will never come back.

The assassin's dagger definitely is not cursed, and makes one of the greatest second weapons in the game. Unfortunately it's also the quest item to become an assassin (walk into the rogue's guild with the character carrying it), and he'll take it from you then, unless you don't have the patch and are prepared to exploit a bug.
Ignore his sending you to the boogre caves, and keep talking; he should give you the dagger.