SPOILER.
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Ok i looked in my inventory and there was the diary of sarevok saying about bahl and all and i wondered where i got it coz i dont remember picking it up!

Thats strange! Maybe it is given to you, I don't know.

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"No!" shreiked Tyrion. Though the sword felt heavy as a fallen tree he lifted it. He saw the deamon look up, astonished. He brought down the blade with the awesome majestic power of a falling thunderbolt. The burning blade caught the deamon right in the centre of its forehead. The deamon's head fell apart under the force of the impact. The deamon N'kari sank to its knees, dead.

-Tyrion at the battle of the Blighted Isle.

I believe you get that in the Iron Throne --- top level --- off of some woman.

Yea, you got it from Cythandria at the Iron Throne base.