The Griffin Steel Sword is a Steel Weapon that is also a piece of Witcher Gear. This particular weapon's diagram can be obtained from the Side Quest: Scavenger Hunt: Griffin School Gear, along with the diagram for the Griffin Armor and Griffin Silver Sword.

How to Find the Griffin Steel Sword

The diagram for the Griffin Steel Sword is found in the abandoned watchtower of Hindhold, located just to the East of the Novigrad Border Post, sitting on the edge of the Pontar river. When crossing the marsh and broken bridge to the main entrance, you'll be met with several level 9 Endrega Workers and a few level 11 Endrega Warriors. Clear them out and head through the ruins and up the stairs to the top.

Alternatively, you can look for some scaffolding on the side of the watchtower that will bypass the Endrega. At the top of the watchtower, you'll encounter a Harpy nest, with a horde of level 7 Harpies flying about.

Force them to land with crossbow bolts and destroy the nest, then enter the ruined room on the left to find a corpse and the diagram, as well as yet another note with more clues as to the final resting place of the Witcher.