So im playing through Tribunal and so far I have only done the temple quests (Almalexia, Hler and Drin) and i got to the forging of Nerevar's blade quest. I already got the two first pieces (the one Almalexia gives you and the one at the museum) but when i go to Karrod and duel him for the blade, he will speak to me, but when i mention the blade it says: "[Karrod says nothing]"

Any ideas?

The fact that you have only done the Temple quests is the problem.
You have to get Helseth to set-up the duel (which is its own seperate quest that he initiates) and after that quest is finished (by talking to Helseth after the duel and getting a reward) Karrod will then speak to you about the sword.

Otherwise, the piece of broken sword is on Karrod's body, so you'll have to kill him.