Sacred Arms, items so important and powerful that no one has ever taken them before until the MC surprisingly comes along. Seems legit.
Nobody has the stats to equip them. And when a hero comes along who can use them, he takes those arms, does great deeds, and puts them back when he retires.
They might also be picky about their user, and if the user turns to dark side, they disappear and reappear in the temple.
The strange part is, there were no priests in that temple. Maybe the MC's slave girl was supposed to be one, but somebody raided the temple and enslaved her?
Alternatively, the temple is only accessible when the demon king has appeared. The rest of the time, it either does not exist, or is protected by powerful wards, such as Mist of Totally Getting Lost.
How to see is the sword one handed or two handed? I got confused because the sword actually big enough for two handed.
Length of the blade and length of the handle. These are the main criteria. Balance might also be different.
I am assuming the MC, as a master swordsman, has a good eye for swords and can tell the difference easily.