If you kill Arbaziloth 10 times on hard mode (you can't let The Forgemaster die), you'll have access to the Doomforge's reward room, where you'll be able to choose 1 of the weapons. In addition, you can drop them if you kill Arbaziloth on easy mode (you must kill The Forgemaster).
As for whether or not it's worth it to permanently imbue them, I think it depends on your level and financial situation. If you decide to drop all the items, you'll spend a lot of time collecting them. The items needed to summon the bosses are rare, and you still need to be lucky enough to drop their skin. In addition, you'll need to collect 500x demonic matters, which are also rare. If you decide to buy all the items, you'll probably spend between 30-40kk, or maybe even more depending on the value of the items in your game world. Depending on your level, I would say that it is worth investing this money in a stronger weapon from Soul War or Rotten Blood.
Of course, there is the advantage of having a permanent imbuement, but as I said, depending on your level, the amount of gold you will spend on this imbuement will not have a big impact on your finances. And at some point, you will prefer to have stronger equipment, to increase your damage and consequently the XP you gain.