???
Steel can melt
Gold can melt
Concentrated energy technically can't melt, but when in a skinned vessel yes it can (unless the souls give them certain special abilities but idk)
Well, the sword can also kill Gods in its base form and was forged in Hell, if you need a God-killing sword, you can't have it melt, now can you? Anyways, yeah, the souls make it immune to melting and any sort of damage for that matter.
