You’ll need all three forge stones for these. You get them so late in the game you may as well wait until you have the 50,000 gold achievement, as well as "minion Lover" (for the gems). Neither are required to finish the game and they cost a great deal. Just save before buying them to unlock the achievement and reload. You will need the infernal helmet (as well as all command upgrades) for the perfect horde achievement. In total you’ll need 175 Browns, 100 reds, 100 greens, 75 Blues, 16,000 gold, 90 gems and 5 dark crystals.