The Unholy Reaver is indeed undroppable, unless you've installed the "Anti-Paladin kit" component of the Tactics mod--which you say you have done. So the fact that the sword exists is normal.
What is not normal is that you found the weapon in Watcher's Keep, or that it has no Description. Try uninstalling the Anti-Paladin component and then Reloading that fight with the Githyanki to see if that changes anything.
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Volothamp's Comeuppance
Everything you ever needed to know about the entire Baldur's Gate series......except spoilers.
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