Ore (鉄鉱石 Tetsukouseki, lit. Iron Ore ) is a Key Item in Knights in the Nightmare that is found hidden within Scene 9 regardless of the story path. It belongs to the NPC Gene, who is found in Scene 12.
To obtain it in Mellia's route, the treasure chest has to be broken for the Foreign Gun first and respawn. This is not necessary if Walder has been kept in the party from the previous playthroughs.