Hellkarium is an extremely rare metal in Aincrad that is known for its extreme strength without sacrificing speed and flexibility. The material is present in both Sword Art Online and New ALfheim Online.
Hellkarium is the strongest material in the game, and also the rarest. So rare in fact, few 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. However, there are some weapons of made of Hellkarium that were not crafted by either of them.