Spoiler...
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The Mace of Disruption is found in the Vampire Lair, in a pool of blood. You can only get access to it in Chapter 3 though.
Whether it's worth upgrading - well, it's up to you. Personally I'd say go for it. The upgraded mace bestows Negative Plane Protection on the user when equipped, and I think against undead the monster has to make a save or be utterly destroyed in one hit.
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
