Overlord II

To forge a weapon you must first go to the forgery in your tower via the left transportation platform across from your throne.
Once there walk across to the lava pit to start forging.
You will need to sacrifice minions, gold, pink crystals and dark crystals with the number of each varying on what weapons you choose.
This is easier done nearer to the end of the game when you have pleny of gold and crystals.

You can forge a weapon in Netherworld Town in Foundations, there you will have the option to forge weapons and armor, for that you must get Forge Stones throughout the game, each will unlock a certain amount of armor and weapons to be forged, to forge a weapon you must sacrifice some minions and spend a pink gem, this pink gem can be obtained by killing elves and gnomes, from time to time they can drop, there are also crystals on the floor like this one in Nordberg Sanctuary, there are some weapons that are needed Dark Crystals, if you take them all you won't have to worry because you can use them to level your minions and forge all the weapons in the game, I recommend that you forge The Reaper, as soon as you enter Empire Sewers you will have to go through a leave alone and this weapon will help you a lot because it can steal the enemy's life.