[Vicious Love]

Also, bear in mind that many of the units killed by the helm would have been killed anyway, either by hurling themselves at a fire shield, freezing to death, being souldrained, or being picked off by a spare attack when they're the last unit in base contact with the carrion dragon.

Edit: Oh, and do factor in the forging time, plus the fact that gems are often worth much more than their alchemy value, as well as the fact that Dominions isn't exactly short on summons, spells and items that give you a 100% return on investment in a full 20 or more turns.