The Amulet of Vathelos is a mystical relic that was stolen by Con, Conan's grandfather, from a Stygian cult who worshipped a demonic entity known as Azaroth, and were in service to Wrath-Amon.
The Amulet has mysterious powers, and protects the wearer from certain malevolent magicks, as Wrath-Amon's attempt to turn Conan to stone failed because he was wearing the amulet. The extent of the amulet's protective powers are unknown, as Conan has been subject to all kinds of magicks used against him and the amulet did nothing.
It is later revealed the amulet was created by a warrior-wizard named Vathelos, who used it to absorb the wicked power of Set. After purifying the power it had absorbed, Vathelos used the power to undo much of the evil magicks done by Set and the Serpent Men.