Nilfgaardian longsword is a steel sword in The Witcher 3: Wild Hunt that comes in 2 different forms:
Nilfgaardian longsword (crafted)[]
Crafting Requirements 
Min. Required Level: Amateur blacksmith
Nilfgaardian longsword (common)[]
In White Orchard, one such sword can always be found in the big chest next to the grindstone in the Nilfgaardian Garrison, but the soldiers there will attack Geralt for stealing it.
It can also be found as random loot or purchased from the following merchants:
- All blacksmiths not in White Orchard, except the one at Ardaiso Quarry
- Merchant at Calveit Watchtower
- Merchant at the Isolated Hut on Hindarsfjall
- Merchant by the road west of White Eagle Fort
- Weapons merchant in Nilfgaardian Army Group 'Center' Camp
- Zoltan Chivay during The Battle of Kaer Morhen (dependent)
- Merchant in Stacheier