Gromril is a rare and incredibly durable metal in the world of Warhammer Fantasy, acting as the setting's equivalent to Mithril or Meteoric Iron for more realistic fantasy settings. Dwarfs are known for guarding the secrets of this mineral very jealously, as they are the only ones capable of mining and shaping it, thus meaning that many examples of gromril weaponry or armour are mainly those found in Dwarf possession with a few very strong exceptions (The Runefangs and Karl Franz' Armour of Meteoric Iron come to mind here) gifted as symbols of great respect.
It was originally found when a meteor crashed into a great volcano. As the Dwarfs realized the value of this mineral, they decided to form a settlement around the valuable metal. This would come to be known as Karak Varn, and as with many other holds, it was lost with the Time of Woes in -1499 IC. There were many quests to reclaim the Karak, chiefly to reach this coveted metal.
Durability: As befits any legendary metal, Gromril is the sort of metal that is impossibly difficult to break. It happens to be stronger than any other metal in existence. Alloys aren't much of a thing as any Dwarf worth his beard treasures the purity of Gromril above all else, and there are other qualities that require as pure a composition as possible. It is possible that for this reason, Gromril can only be used make plate armour, as it is said only Grungni can make chain mail out of Gromril. Presumably, this is due to scarcity.
Rune Retention: For whatever reason, Gromril tends to be the best metal for applying runes. If applied on any other metal, the rune either doesn't work or twists and cracks until the rune's power causes it to explode. Gromril, however, keeps its shape once runes are applied to it. This allows the Dwarfs to enchant their arms and armour with various unique properties, and was even used to forge the magical Anvils of Doom.