You can purchase the Fusion Cauldron from the Workstation Furniture menu. Like other workstation, you can only purchase 1 Fusion Cauldron.
The Fusion Cauldron comes with 3 item combining cards and may have a max of 5 item combining cards. In order to unlock these items combining cards, upgrade your Fusion Cauldron. Having a higher level Fusion Cauldron will allow you to fuse higher level items as well as a greater success rate for better quality fuses.
Fusion allows you to combine different items to make new/better quality items.
This is extremely helpful for items that require high-quality items as part of the Blueprint.
You may purchase Upgraded Fusion slots to allow multiple fusions to take place as once with Gems. The prices are as follows:
2 slot: 1000 Gems
3 slot: 3000 Gems
4 slot: 5000 gems