Hellkarium

Hellkarium is an extremely rare material in SAO that is known for its extreme strength without sacrificing speed and flexibility.

Properties
Hellkarium is the strongest material in the game, and also the rarest. So rare in fact, that only 3 weapons are actually fully made of Hellkarium. The most that is usually done with Hellkarium is reinforcing other weapons, adding to durability and strength significantly, but not reaching levels of a pure Hellkarium blade.

Hellkarium smithing is extremely difficult and requires complete and total mastery over the forge, with swords being the most difficult weapon to make with the material, as well as being near impossible to find enough material to forge the blade.

There are only two blacksmiths in SAO known to be able to make weapons with Hellkarium, being Hephaestus and his apprentice Antinomy.

Hephaestus

 * Lamentation
 * Majora's Wrath

Antinomy

 * Guren