Once you've accessed the Bloom Palace inside your HQ at Fortuna, Tiki will be able to help you perform the Carnage Unity ritual. This allows you to create new weapons or Carnage by using the Performa dropped by enemy mirages. It's an integral part of the game and the only way to learn new skills so upgrade your Carnage often, preferably after mastering the skills they can inherit. Please refer to the dedicated section for the individual Carnage list of the game's characters.
Weapons are called Carnage in the game. Carnages have different stats like attack boosts, resistances, and skills that can be inherited. Once all skills are inherited from a Carnage, it becomes "Mastered" and can be replaced with another one.
You can't buy or obtain Carnages via store or item loot; to get new Carnages through the ritual called Carnage Unity. This requires Performa, dropped essences of defeated enemies. You can also obtain more Performa through Session chains. Since these are enemy drops, you can always check a Carnage's detail screen in the Bloom Palace to know what types of Performa needed for that Carnage. You'll be able to see the type of enemies and their locations in the same screen as well.
There will be times where you'll learn the same skills from other Carnages that you already have. In this case, the skill of the same name will be powered up instead and will have a "+" sign beside its name. For example, if you already have Dia and you learned another Dia from another Carnage, your Dia becomes Dia+1.
Powered up skills become more powerful (amount of HP healed increased in healing skills) and their EP cost will be decreased as well. There are exceptions however; this rule won't take effect certain skills, especially the passive ones and those that change the affinity compatibility.