The Mail of Olethros was an enchanted chain mail armor crafted by the Maiar Olethros during either the late First Age or early Second Age. One of the Treasures of Olethros, it is an extremely valuable artifact.
Olethros' Mail is a sleeveless chain mail shirt, appearing to be crafted out of intertwining silver rings. The mail was fitted to be worn by the average-sized man, though was also capable of being worn by taller men and elves, as well as shorter dwarves, but was ill-suited to be worn by hobbits or orcs.
The shirt was enchanted so that the wearer would not tire in combat, allowing them to fight perpetually, even for days. However, the mail had a weakness in the wideness of the rings; allowing arrows or small daggers to slip through easily. This was intentional; as Olethros wished to emphasize that one does not rely on the power of the Mail to further their own agendas.
Olethros himself crafted the mail, as well as all of his artifacts, during either the late First Age or early Second Age. It was gifted to the Kingdom of Rhovanion and was a symbol of the royal family.
At some point during the third age, the Mail was lost. Historians debate whether this was in TA 1612, during a small uprising, or in 1636, during the Great Plague of Rhovanion. Regardless, by the year TA 1700 it had wound up at Isengard's tower of Orthanc, where it would later be recognized by Saruman. It is believed that Saruman outfitted one of his Uruk-Hai captains with the armor, for it was discovered amongst the corpses after the Battle of Helm's Deep, where Gandalf the White took possession of it.
It is unknown what became of the mail after this point.