The Black Spear is a wyter-like object that helps creating a tribal and clan unity among the Colymar. Originally it was the weapon of chief Colymar, probably with ties to Argan Argar (the troll spear god) and a symbol of authority even towards the Kitori.
Gaining the Black Spear basically gives Argrath the support and leadership of the Colymar tribe – the first and one of the most powerful tribes of Quiviniland. It is a “back to the roots” quest for the adventurer who spent a lot of time gallivanting around in Praxian exile and sailing around the Homeward Ocean for most of his adult life, the proof that he still is one of the hillbilly guys despite his strangeness and his outlandish companions.
Bringing back (or introducing) an extinct beast of prey appears to be a side effect of the Lady of the Wilds quest that seems to be part of the King of Dragon Pass setup. Bringing the Aurochs has a double connotation through Urox’ anti-Chaos abilities, strengthening the land against the coming onslaught of (Lunar-stirred) Chaos.
All of this pertains to sovereignty and unifying his followers, so that’s the kind of magic I would assign to the spear.
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