Skellige longsword is a steel sword in The Witcher 3: Wild Hunt. It comes in the following forms:
Skellige longsword (crafted)
Crafting Requirements 
Min. Required Level: Amateur blacksmith
Skellige longsword (basic)
One is always found with the Trial of Dexterity completion token atop Yngvar's Fang. It can also be found as random loot or purchased from the following merchants:
- All blacksmiths in Velen, Novigrad, Oxenfurt and Skellige
- 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
Gallery
