Those poor cyclops...
Start it out as an artifact. I don't think many of the standard D&D magic weapons can survive being submerged in magma for x generations. Even the BBEG couldn't destroy it. Best he could do was deliberately get hit with it and not let it return....
Give it a special purpose (save the world, destroy evil, something) with a smite special property.
Treat it as an exotic dagger. Those who can wield the weapon propery can control the Glaive as if they were using
Telekinesis (20th level caster) at will, but only on the Glaive itself. Give it a +5 while being used in this manner, and the base damage increases to 1d6 due to the spinning action. I'd also have it ignore hardness and be treated as good when figuring DR (it cut through a magically enchanted crystal wall [albeit slowly] and severly damaged a previously untouchable BBEG).
In short:
Attack bonus: +5 enchantment + Int modifier (+5 enchantment + Str in melee)
Damage: 1d6 +5 enchantment (1d4 +5 +Str in melee)
Range: 1200 feet with no range penlaty, per telekinesis
Weapon is treated as Good when considering DR
Weapon may be controlled as if the wielder were using Telekinesis, as cast by a 20th level caster, at will
Smite Evil X/day. This ability may be used when the wielder controls the weapon via telekinesis
My two cents...
