The following text is a guide to getting the mace and spoilers are not marked for the sake of comprehensiveness, you have been warned.
A key element is Ravel's maze, be sure to be sleazy enough with her in order to get more than one Black Barbed Seed, or you can just get one and not use it to create a wand, the choice is yours as long as you keep at least one Black Barbed Seed after the maze.
Another thing you must do in that maze is talk to Ravel about her incarnations which you have met during your travels through Sigil, especially Mebbeth.
Once you're back in Sigil go talk to Mebbeth to find out she's dying because the primary Ravel has been killed. Mebbeth will ask you for a Black Barbed Seed and ask of you that you seek out her daughter - Kessai-Serris the "prostitute" in the Brother of Slaking Intellectual Lusts in the Clerk's Ward.
(You must have completed the dreambuilder in the Godsman Forge in order to proceed.)
Kessai-Serris will have a brief telepathic conversation with her father and then offer you dream guidance, if you accept (and you should) she will tell you she needs a day to prepare, so you'll need 24 game hours to pass before you can continue with this quest.
Once enough time has passed you can talk to Kessai again to go into the Forge, where you will be separated from your party and get an explanation on how stuff in your dreams works (Anything you drop or consume stays that way).
You will get a short glimpse of the Practical Incarnation's party (The blind archer is depicted as a zombie
) before they went into the Fortress of Regrets and some dialogue of said incarnation as he puts stuff in the closet in your private room in the Civic Fasthall that you couldn't open (I'm talking about area 4 in the section 'Civic Festhall 2').You will now find yourself in the north western corner in UnderSigil and just to the south east are 2 Greater Glabrezu which you must defeat, ALONE.
Luckily there's an exploit you can work to do this otherwise nigh impossible task, this will be described below.
If you get them (or one of them) to chase you north west there is a point the Glabrezu cannot pass because they are simply too big :laugh:.
Now you can either spam spells as a mage (be advised they are pretty much immune to magic, I cast Mechanus Cannon and it didn't even scratch them, Meteor Shower works good though). If you're a thief I can imagine you can go around the South West corner to activate stealth and backstab them repeatedly. Fighters have the least luck as you pretty much have to engage them in melee combat and they are TOUGH.
You best option is to stand back and use that Black Wand you have from Ravel's secret garden to spam Black Barbed Curse repeatedly, so make sure you have enough Wands.
Once both Glabrezu are dead (70K exp each for you alone since you beath them without your party!) the dream will automatically end and Kessai will tell you she is leaving the "Brothel" to explore the world.
Now you can go back to your room in the Civic Festhall and open the closet to find the Heaven and Earth Maze, a Glass Eye and a magic quill that will have some interesting things to tell you.
So here you go, perhaps you can add this to the Walkthrough to prevent people searching for the mace from farming needlessly in UnderSigil.
I am playing the fully patched version of the game, including the 'Unfinished Buisness' mod.
I hope this info has been useful.
PS: I have bought the Eye of Vecna from Vrischika's weapon and charms, you can add that to the walkthrough as well if you want.