A Rogue Drayk Tendon is a special ingredient used to create special items. It only appears in Geneforge 2 and there is only one of the item.
Sharon, from the Sharon's Grove location in the second game, requests the player bring her one of these in conjunction with purified essence and perfect crystal so that she can fashion for the player a powerful ring. The ring adds +3 to Leadership and Mechanics and +2 to Dexterity.
The Drayk Tendon can be found in the Shaper's chambers in the Clockwork Maze.
The Shapers outlawing the creation of Drayks may be a reason why this item is rare.