Mantle is a skill in Fire Emblem: Path of Radiance and Fire Emblem: Radiant Dawn. In both games it is an NPC-specific skill. In Fire Emblem: Path of Radiance, Mantle is only used by Sephiran, which negates all damage done against him and prevents Critical hits.
In Fire Emblem: Radiant Dawn, Mantle is used by bosses who appear in the final chapters of the game. It negates all damage by weapons not blessed by Yune, prevents Critical hits, disables all enemy's combatskills and restores health equal to the user's luck each turn.