Talking about inefficient cards in Fallen Kingdom, Hellforged Weapon is really inefficient, often doing more damage to your units than it actually does to enemy cards.
However, every aggro deck needs to threaten assassination to work and this often gets that job done.  The 3 main uses for Hellforged Weapon are:

- Explosive focused damage with Undead Archers.  Attack, have your Undead Archer die and be replaced with another and then attack again.
- Another sacrificial outlet (although this can be unreliable).
- It can be somewhat economically efficient with an Undead Carrier, as it allows you to attack a high life target, kill it and then it protects itself with the new carrier, potentially allowing you to repeat the process next turn, unless of course your opponent finds a way to kill the Undead Carrier.

One interesting interaction is with a Hellfire Cultist, where you usually are hoping the Hellforged Weapon kills the Cultist after the attack for maximum damage.  If it doesn't roll enough specials to die, it probably did quite a bit of damage and the weapon was worth it anyway, and if it does roll enough specials, it died just as you wanted.

Also notice that the strength bonus stacks, but the ability does not, so you can give units both Hellforged Weapons, granting +4 to attack strength without doubling the damage received by the unit from Cursed.