The Diablo 2 Wiki
The Diablo 2 Wiki archive is available for anyone looking for information on Diablo 2. All information here is pre-D2R but contains a lot of useful information that is still relevant. Updated sections for new D2R features can be found on the PureDiablo Diablo 2 section

Khalim

From Diablo 2 Wiki

Khalim was the last true Zakarum priest who battled against the corruption of the Act 3 boss. He was murdered and then butchered, with various portions of his body hidden in strongholds across the land.

Khalim's Flail and body parts.

In Diablo II you must gather them all and then combine them with the Horadric Cube. As you can see in the image here, you need to gather Khalim's Flail, along with his heart, eye, and brain, and then click the Transmute button, which makes a pretty little graphical effect, and leaves you with just the Flail, which is slightly altered in appearance. It's quite a useful weapon, but you can't use it for long, since you need to destroy it when you smash the Compelling Orb, breaking the spell that has the enemy Zakarum on the surface attacking you. The destruction of Khalim's Will is also required to open the stairway down to the Durance of Hate, where you must go to finish Act Three.