Black Crystal Blade.

What items do you need to make it? (I can't get to El Nath at the moment and I would like to know if I need a Zakum Diamond or not before I go there. If Zakum Diamonds even still drop.)

Zak Diamonds do not drop anymore.

If memory serves me correctly it was a crystal blade and a zak diamond to make it, possibly a few other things thrown in there aswell.

Edit:

First talk to Adonis and present him the following items to make the upgrade.
Crystal Blade,
One-Handed Sword Forging Manual
One-Handed Sword Forging Stimulator
50 Dark Crystals,
and a Zakum Diamond

Taken from the maplewiki