HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

It's not entirely clear how the developers originally intended this to work. The property itself was likely not added by accident and is probably not a bug (just like the fact that psychic elementals deal half damage to creatures immune to mind control isn't a bug), but it's possible that it was mistakenly assigned to the wrong artifact or was overlooked when artifacts were assigned prices and levels – it's hard to say for sure.