WoWDB

Sul'thraze the Lasher is an epic 2-hand sword with a unique appearance in the game.  It has very dark coloring and two skulls that constantly orbit the blade like an  Unholy Weapon enchantment.  Because of this, noot only is it cool to look at but it also has the effect of hiding any enchantment applied to it.  (This was a great perk when the transmogrification feature was added but before we gained the option of hiding enchantment visual effects on our weapons.)

The sword is created by combining  Sang'thraze the Deflector and  Jang'thraze the Protector.  Sang'thraze drops from Antu'sul (the boss found in the basilisk area of the dungeon).  Jang'thraze drops from Chief Ukorz Sandscalp (the boss whose uphill path is protected by a gate).

Antu'sul is straight-forward enough: run to his location and kill him.  Chief Ukorz Sandscalp is a little more involved; you have to do the temple event by freeing the prisoners and killing the mobs that spawn, followed by killing the formerly-imprisoned NPCs (they have it coming, trust me).  Now the path to the chief is open and you can go kill him.

Rinse and repeat until you have both blades.  Once the game detects that you have both swords in your inventory, a new use effect appears on Jang'thraze that will allow you to combine the swords.  Sang'thraze and Jang'thraze are consumed in the process of forming Sul'thraze.

There is a new Legacy achievement on the 7.1 PTR for acquiring the weapon.  This has lead to speculation by the community that a change may be coming to the 5-man dungeon that will cause Sul'thraze to become unobtainable.  I don't personally know if that will end up being true, but I farmed the swords in a single night of resetting Zul'farrak.  I strongly recommend looting all the sand graves, by the way - I looted some desirable uncommon-quality gear and even landed a vanilla epic BoE staff from one of the troll zombies.  A night well spent!