Your metal-based spells are more difficult to resist.
Benefit: Add +1 to the Difficulty Class for all saving throws against spells you cast with the [metal] descriptor.
Spells with the metal descriptor are those that affect objects made of metal or directly create, utilize, or manipulate metal in some manner. Several of the spells in Path of Iron have the metal descriptor added. Some abilities, feats, and magic items found in Path of Iron, such as the Metal Focus feat, rely on the metal descriptor for their function.
If you use the [metal] descriptor in your games, several existing spells gain the metal descriptor, as listed here:
bullet ward, chill metal, fabricate bullets, heart of the metal, heat metal, iron beard, iron body, mirror polish, molten orb, pellet blast, repel metal or stone, rusting grasp, silk to steel, silver darts, transmute metal to wood, wall of iron, wreath of blades
While these are the only spells that directly utilize metal found within the Pathfinder Roleplaying Game core series, many other supplements not found in these books contain spells that, if included in this book, would likely be given the [metal] descriptor. Consider giving the new metal descriptor to spells featured in your favorite books that utilize metal in some manner to further expand upon this new ability.
Source: Path of Iron
Path of Iron, © 2015, Ascension Games, LLC; Author Christopher Moore