Stormtree probably doesn't have a special TreasureClass...but look in Superuniques.txt to make sure. If he does, you can manipulate that Treasure Class in TreasureClassEX.txt if no one else is using it.Otherwise, make your own treasure class for him.
So...first, read the treasureclass tutorial here at the Keep. Next, do this:
Create a new row at the bottom of TCEX.txt. Call it Storm or something like that.
Make the picks -1
Make NoDrop 0
In the first item colum, put the itemcode (found in weapons.txt) for either Khalim's Will if you want the complete item or Khalim's Flail if you plan on making the organs available somehow in that same Act.
Make the Prob for that item 1
Make sure you fill in other important columns. For example, I think you need a 0 in the last column or the game will crash.
Save the file.
Go into Superuniques.txt.
For all three treasure class columns (Normal, Nightmare, Hell) put the name of that flail-dropping treasure class (Storm).
That's it, I think...but we just had our office party so I don't vouch for my complete sobriety and correctness. 
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- JEFF
"A hero is no braver than anyone else. He is only brave five minutes longer."
For my latest mod (for LoD/expansion), download the Preview of Sanctuary in Chaos vesion 1 - Or for old stuff (for classic D2), visit my orignal mod site at www.planetdiablo.com/sanctuary