The Roc Feather is a key item in Golden Sun: Dark Dawn, which means it cannot be dropped or sold. Its description reads as A giant feather from the legendary Roc. It is obtained at the top of Talon Peak when the player uses Slap Psynergy on the dewlap of the Legendary Roc.
Obtaining the feather is the initial reason Matthew, Tyrell and Karis set off on their adventure. Tyrell broke the soarwing which Ivan built for Isaac; therefore, they needed to find a Roc Feather in order to build a new one.