SPOILER
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In chapter 3 ,after either paying for the Shadow Thieves or for Bodhi, you'll be allowed to enter a secret section of the Graveyard District tombs - to help Bodhi or to fight her and her gang of vampires depending on your choice. Either way, you'll come across a pool filled with blood (not the one surrounded by spikes) and you'll be able to grab a mace into it (be careful, it will take away 10 hp from the character who enters the pool). Then take it along with the illithium, go to Cromwell's house in the Docks District and then ask him to upgrade your mace. It's expensive, but worth the effort!
Cheers,
Kitty