Helliron is especially good against anything supernaturally empowered to represent the concept of Good such as angels, paladins of good deities, clerics of good deities, and so on. It's also capable of harming creatures that can't be harmed through normal weaponry, serving as either cold-forged iron or silver. Also, somebody it kills that hasn't already been claimed by another power, be it demon, devil, or deity, may find their soul awakening in an Abyssal, Hellish, or otherwise Fiendish mine and forced to work on mining more Helliron from the ground. Most deposits are in Hell, hence the name, but that doesn't mean every deposit is. It also won't be as effective on anything supernaturally empowered to represent the concept of Evil, such as demons, devils, paladins of evil deities, clerics of evil deities and so on.
It's also easy to enchant with flame or ice magics, and may manifest such abilities on its own, especially when wielded against angels in particular, but never both. Such abilities will not effect demons, devils, or anything else of that sort; evil paladins and clerics however aren't so lucky, though it's still less effective than a plain steel weapon of that type with that kind of enchantment.