Mace of Disruption +2 (Spoiler!)
I have a good news for you, o'mighty roleplaying advanturers.
I found a legit way to get the real Illithium to Sir Sarles AND also get it back for the Mace
Summon a Pit Fiend (using the Spell Gate, Level 7, without using Protection from Evil and enjoy the killing without losing any reputation
The whole house will turn hostile however, but I guess that would be normal. Nobody is hostile outside the house. Just make sure not to kill anybody 
Enjoy your 200 pounds of real Illithium from Sir Sarles body.
I certainly would be mad at you if you came to summon a Pit Fiend in my house!
I found a legit way to get the real Illithium to Sir Sarles AND also get it back for the Mace

Summon a Pit Fiend (using the Spell Gate, Level 7, without using Protection from Evil and enjoy the killing without losing any reputation


Enjoy your 200 pounds of real Illithium from Sir Sarles body.
I certainly would be mad at you if you came to summon a Pit Fiend in my house!
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Comments
He's one of the bartenders who has also a shop. Like Bernard in Copper Coronet. Some nice other items too, like Reaching Ring (additional arcane spells from 5,6 and 7th level). Besides - who cares? It's Sigil. A place where you can find celestials and lower demons several feet from each other.
A better question is why Cromwell needs 200 pounds of pure illithium to enhance a mace, especially when the weight is unchanged from the original Mace of Disruption. I think Cromwell might be pocketing some of that ore for himself, greedy bastard.
Instead, expect suffering.
You will suffer! You will ALL suffer!