Same story here.
I found this halber in SoA and tried to sell it to each merchant encountered. Nobody bought it! I was so curious to find out what is it all about, that, after finishing SoA, began to sell it in ToB
With no success though
If anybody know about some quest connected to this thing, share with us, please!
Now I believe that it is just game feature - may be former owner of the halberd was not legitime one and the thing was known as stolen...
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Tanda,
with best regards from Guild of Assassins