- Little Rogue and The Slanderer are two completely different items of the 2-piece Istvan’s Paired Blades Set.
- D3 does not change an item into a completely different item. (Regardless of the Recipe).
- The only Cube Recipe that can change an item to a different one of the same Set is Skill of Nilfur. (And it will only do that for Sets that have more than 2 pieces).
Unless you video recorded your experience, we can only assume you had two Little Rogues to begin with. I would suggest you keep the better of the two Primal Swords you now have and salvage the other one. At least then you can recover 55 Primordial Ashes.
Finally, you can farm The Slanderer. A well rolled Ancient one could be better than a poorly rolled Primal one. And, if you’re lucky enough to find multiple Slanderers, you can Reforge the worst ones in hopes of getting a well-rolled Primal one. (If it means that much to you).
Whatever you decide, good luck and good hunting! 