Can anyone tell me were to get the design for the darkness weapon( Ragnorak..

I'm pretty sure it's found at Renaba Ra Te (if that's really what it's called...) and to craft it I think you need a Dark Sphere which you can by from a merchant in your village for about 17,500 gil, and to craft it you need about 50,000 gil! The Ultima weapon is the same price, plus it knocks an enemy back when you hit them, so I would craft that one instead.
P.S. It takes 6 years minimum for the merchant in your village to start carrying Dark Spheres.

Sword master is wrong you need the ancient sword which you can get at river belle path for a certain amount of points (check your game boy for your bonus it's easier if you end up with the bonus inflict damage you get at least 2 to 3 hundred points each time) The cursed crook which you find in goblin wall just like the ancient sword I mentioned before, the dark weapon design found in Rebena te ra just like the cursed crook and the ancient sword and you also need orichalcum and 5000 gil the sword can be made at marrs pass and alfitaria. *Ragnarok's focus attack forces you to forfeit half of a heart to use it but it is worth it because it takes down most enemies in one or two hits. I should know this works because I have the weapon along with the gaia plate, ring of incincibility, and the chocobo shield which are the best weapons and armor for a clavat.

"6 years for merchant in your village to start carrying Dark Spheres."

Thats not true. The merchant starts carrying dark spheres when it's upgraded to third level. Your village blacksmiths merchants etc are upgraded by having a character talk to his father twice in a year (next year it's upgraded) or so I'm told by guides for this game. But I know I've seen it in year 4 anyways.