How on Earth do you get it, and then forge it in the black forge?
Keldon was telling me to take from the tower to create a weapon, but I could not figure out how to do it. Any suggestions? Thank you.

The Onyx golum drops it after you kill him. He is found guarding the stairway to the Hall of Heroes.

Ohhh is that the big big big guy you fight before going to where the ghosts are? Argh I killed him and did not find anything..hmm...

I am playing again on Hard now, and I will be sure to look for it now.

Thanks man!! Appreciate it!!!

Jon when u said how do u get the onyx sword and then forge it in the black forge, what did u mean? is there a way to like improve it or sumtin? plz help me im so confused

Well, when you talk to Keldon upstairs in the hall where the ghosts are, he says it needs to be forged in the black forge, which is down the next floor. SO I assume that you can improve it, or do something to it. I really do not know, as I have never had the sword before!!!

When you kill that big BIG dude, just walk around his body taping the action buttin, its kinda hard to see cus it kinda looks like the floor. Its a really power 1 handed sword. its got no mods but it think it ignores the last bosses health, cuz all me other weapons only do about 30, but this beast unleashs 90-110 and 300 critcials. well thats with me.